Wow. That is by far one of the more deep situations I have read in a long time. On one hand, you could say that you would not want to live through such hell for 20 years, but on the other, you could also say that you're not that old. I mean I could live 20 years in one of those round things, with all the essentials. I know my social behaviour would eventually die out, but with the right clones and such and certainly with the right items, I could easily live through the 20 years.
A bullet and a gun might seem plausible, let's say, if I were 60 years old. Because I would know that there's a good chance I would end up dying, from the lack of stimulation anyway. Now let's say I'm roughly 20 years old, I could take that.
A, I would definitely choose A, there's just no other way that I could or would ever pick the gun and bullet.