It isn't any of them. It can't be B or C because the probability of choosing either of those answers is 1/4 which isn't equal to their probabilties. That means it must be A or D. If the answer was 25% then the proability of guessing it correctly is 1/2 which isnt the answer to A or D.
I'm a total retard when it comes to this kind of stuff, but surely (as the second poster suggested) there is no correct answer because the question posed doesn't have a specific answer in itself.
A question can only be answered if it actually has an answer. So in order for the posed question to be answerable, there would need to be another question before the posed question. So, for example, let's say that the imaginary first question is 'what is 50% + 10' which would make the answer 60%, which is poll option C. And therefore the answer to the posed question would be that you have a 25% chance of getting the correct answer. But because there are two 25% options (A and D), the answer to the posed question would is 50% (B).
But if the posed question isn't preceded by a prior question like the one that I have just made up, then it can't be a question in itself, because it can't have an answer. So therefore there is no answer.
(I have no idea whether any of that made sense, I only managed to scrape a B at GCSE Mathematics)