Crazy multiple choice question

[lavoisier]

Hi everyone,
a friend once emailed me a crazy multiple choice question. I thought about it for a while but then gave up.
I can't find it anymore, but I remember it was something like:

What is the probability of answering this question correctly?
1) 100%
2) 50%
3) 33%

Does anybody know what the actual question may have been (as I guess it's pretty well known in mathematical circles) and especially, what the answer is?
Thanks!

 

[HallsofIvy]

That is, simply, non-sense! Obviously, your chance of answering any question correctly depends upon what the questioner will accept as the correct answer! I suspect that what is intended is to assume that anyone of the three answers is equally likely to be accepted as the correct answer, therefore, by choosing "at random", you have a 33% (actually 33.333... % of 1/3) chance of being right. But, then that answer has only 1/3 probability of being right!