Ooh, that's a really tough question.
See, a part of me thinks I'd immedietly choose to sacrifice myself. The religious, god-fearing, stupid part of me.
The other part of me thinks "duh, run like fire". The smart, instinctual, I'm more important than the world part of me.
So, what would most likely happen would be that, while I was debating with myself over what I should do, someone would come along and kill me anyway because I was taking too long to decide. But since I never really agreed to the terms, the Yeerks would continue their conquest of Earth.
That is, of course, a worst-case senario.
The worst-case for me senario is that I agree to sacrifice myself on two conditions: 1) I am left alive until every last Yeerk is off the planet and goes back to their homeworld, where upon the Andalites come and destroy every last piece of technology they have and 2) That I get to choose the form of death. Then, of course, the Yeerks manage to build more technology because some Yeerk managed to find an Andalite stupid enough to let them infest him and continue their conquest, skipping over Earth because Earth is now armed to the teeth, due to "Yeerk Awareness" and Homeland Security.
The worst-case for Earth is that I've started carrying a 38 caliber gun and put a nice little hole between Visser Three's (or One, depending on the book and/or whose giving the ultimatum) head. Then I run like hell and shoot as many enemies as I can becuase I don't have the time to morph, whereupon I supsequentially die because some ticked off Yeerk kills me for killing his leader.
I'm starting to think that I like the second senario best. At least I manage to do some semblance of good in that one. After all, in the first I just die, and in the third...well, the Council's still around, so it's not like I've toppled the entire government or something.
Ok...I'm having a little more fun than is normal for most people...I think I should leave the board now...