The question defines itself which is the root of the problem. To answer the question means to define the question in such a way that your answer is wrong. This may lead you to believe that 0% (C) is the correct answer, but then you'd be wrong (since that's the nature of what I just described).
If the answers were considered only by their value (not their letter) then put into a set as Mute would have it then we could just drop one of the 25% answers and say it's 33.3%, but we aren't dealing with a set. We're dealing with a list of ordered pairs and we need to treat it as such.
Now the question itself as I mentioned earlier is recursive in that choosing a "correct" answer affects the correctness of that answer. So the real question that this is asking is "Which answer can you choose to be correct such that the subsequently defined question is correctly answered by the answer you chose to be correct?" If we assume the condition that ONLY one letter (A,B,C,D) can be correct then we can do a case analysis of the problem and hopefully find a "solution".
To begin, since there can be ONLY one answer as per the precondition I defined then immediately A and D are eliminated since calling either "correct" means the other is also correct. This causes a contradiction with the precondition and so A and D cannot be considered for solutions under the precondition. That leaves only B and C as possible correct answers to the question (remember, I redefined the question as stated above). Choosing C to be correct immediately makes you incorrect since saying it is correct that there is a 0% chance of getting the question correct creates a contradiction. Therefore B must be correct right!? But no, choosing B to be correct means that there must be at least 2 correct answers, and that creates a contradiction with the precondition.
What did I just do there? I just proved (by contradiction) that the precondition that ONLY one letter (A,B,C,D) can be correct is not a valid precondition for this problem. That means that there MUST be either no correct answer or multiple correct answers. However, in proving that the precondition I defined above does not hold for this problem, I also proved that no correct answers is impossible since that causes another contradiction since 0% (C) would be an answer. Therefore, the only case for this problem is one in which multiple answers (A,B,C,D) are correct.
Based on the above information, I postulate that the answers B and C are the correct answers to this problem. At first you might think that A and D must be the correct answers since the only logical way 2 or more answers to a question can be correct is by having their value be the same. However, this case causes a multiple answer contradiction since A and D being correct means the correct answer to the question is B. But how can B and C be correct when their values are different? Well that gets a bit sticky (this is where my formal logic begins to break down somewhat). You see, it may at first seem that choosing C to be correct in and of itself creates a contradiction, but this only holds for the single answer scenario. Choosing C alone to be correct should actually answer the question BECAUSE I proved above that there must be more than one answer to the question (choosing only C means you missed an answer). B may also at first seem to cause a contradiction, but again this is only for the single answer scenario. In the 2 or more answer scenario, there must be at least 2 answers. If we choose B to be correct then we can have 2 answers (B and C) and B is therefore correct. Basically by choosing both B and C to be correct you create 2 equivalent states of the question, and in both states, B or C hold to be correct.
Therefore, I theorize that the answer to this question is B and C.
edit:
Wait a second wtf just happened? That just might be a valid logical answer to this paradox :O. I think I just blew my own mind........