It is an interesting question.

Without external stimuli, we would only have internal impressions to rouse the mind and transfer force and liveliness to their related ideas. This means emotions, pretty much. Whatever emotions we felt would transfer the thoughts along related ideas, but as emotions only admit to the relation of resemblance, this relation would dominate our chain of perceptions. I expect we would experience cyclical mood changes, during which our mind considered ideas of things which produced the currently dominant passion, transferring along ideas which are related by contiguity and causation where convenient.

Come to think of it, this description sounds a lot like a dream-state.

Interesting question. I would say option D is the most likely scenario. To keep its self busy and to attempt to ward off insanity, the mind would create a new reality to exist in. A reality governmend by the person's core personality traits. And over time, the subject would likely begin to consider this dream like reality to be the true reality as it thinks about it's prior reality less and less.

That sounds very likely, indeed, but I wonder if the mind would be able to entertain itself like this forever, or boredoom would soon or late hit it. I suppose it depends of whenever human minds can trully create "new content", or if the reconbinations they can do with what they have have limits.

In such a state I wonder how long the sense of self can exist within the mind; how long before the internal voice of 'I' became lost in the mayhem of all that nothingness.

Surely at some point we would lose our sense of self amidst the confusion: without any reference to our physical existence our sense of being couldn't be maintained.

After all, it (we) is only an illusion.

Even without being able to feel sensory input, we could still feel internal impressions, so I think we could still maintain a sense of self in relation to those. For instance, I see no reason to assume we could not feel fear in such a state, and that would give us the context to think "I am afraid."

Its all in the computer tower, but the harddrive is seperate from the mother board.

Hence in a way "thinking" is a sensorial input function aswell.

ran out of time..

i think the mind would go mad. from the time we are born in our mother's womb, we can hear her heartbeat. we grow up felling and touching and being touched by our parents. all our lives we smell, taste, hear, see and feel; so, i would say that our minds wouldn't last long. our senses are our reality. without our senses there is nothing to ground our reality on. in fact, i changed my mind, we wouldn't go mad, we would just cease to exist.

Every description of what anxiety feels like that I can bring to mind appeals to physical sensations. Butterflies in the stomach, elevated heartrate. It leads me to the opinion that the component of anxiety that causes us suffering is our bodily reaction to the doubts and fears cast in our minds.

I guess this post is what comes the closest to reality.
This issue is actually no fun.
Let me remind you that there have already been experiments that investigate on that topic.
For example rooms that absorb any kind of sound.
It shows that people quickly start suffering panic attacks in such a room.
The reason is very simple: Seperation anxiety.
These rooms absorb even the resonance of our own sounds. So even when we think it's absolutely silent we always hear the echoes of ourselves moving, breathing, whatever.
In a room, that has absolutely no sound however our (sub-)consciousness perceives the impression of being completely disconnected from the environment.
Our mind responds with serious nervousness and even panic to this kind of input.

Another inhumane experiment is done to thousands of people who are kept in 'solitary confinement'. People who are sentenced to spend their time in prison in total darkness and with completely no contact to the outside world.
Experiments show, that some people start having severe optical halluzinations even after 12 hours already.
Test persons report panic, depression, any kind of emotional pain a person can go through.
In fact the brain slowly reduces its activity. I saw a report about one guy [edit: a prisoner]who after some years was released, and found himself unable to take part in traffic. To much input, he couldn't process it anymore, even years later.

So you can be pretty sure that a brain who experiences that situation described in the beginning will go through hell before it slowly starts dissolving [edit: decaying].

Our brain is by nature made for perceiving stimulation.
That's an experiment you can start by yourself: Sit in front of a white wall for as long as you can. I know a woman had to do this for days as a spiritual exercise. Believe me, it takes good nervs and a stabile character.

Apart from that, i think a separated brain would still be able to find itself in a condition in which it can have panic not in a physical way, but just knowing that the situation is absolutely alarming.
It would be like "I am not scared, but something tells me that this situation is absolutely scary." Plus "I have to do something about this unnatural situation".
Fascinating in fact.
It would reduce every emotion from the phenomenological level to the mere epistemological level.
A depression would not feel like sadness or tiredness or whatever.
It would be a mere condition of mind.
Except we believe that qualia are not physical but really conditions caused by consciousness.

I'm also wondering about the simple automatic motions that are controlled outside our brain, such as walking and certain reflex movements, which can be controlled by the spinal chord alone. If disconnected from the spinal chord, I'm sure we would still have the memory of walking, but how complete or convincing would that memory be? And, could this be the reason that we find running to be so difficult in our dreams? I had a dream just the other night that I was trying to run from something, but the motions did not come naturally, the memory of very deliberately and awkardly placing one foot in front of the other is still fresh in my mind. Its giving me goose-bumps just thinking about it.

Another thing I'm wondering about is our memories of our physical responses to things like fear and anxiety, and how convincing those might be. To draw once again from common experience, I have had dreams where I was aware of my heart beating very quickly, one dream in particular I was afraid I would have a heart attack. I woke up and was very surprised to find that my heart was beating very slowly, just as you would expect it to while sleeping.

One way to test this, if given the chance, you should try to spend some time in a sensory deprivation tank. That's the closest you could come to replicating an enviroment without another stimuli. Also, one thing to keep in mind is if this is done with the person knowing before hand what is going to happen or if they just wake up and everything is gone. I think that would greatly change the mind's reaction to it.

I had a dream just the other night that I was trying to run from something, but the motions did not come naturally, the memory of very deliberately and awkardly placing one foot in front of the other is still fresh in my mind.