None of the above. Although how you define random can change your answer. I (along with my Math Professors) define it as never reading the question and blindly picking an answer. If you read the question, bias is introduced and your "random" guess is no longer random.

Reasoning:
Each answer is correct 1/3 of the time (25%, 50%, and 60%), add up all 4 probabilities and you get 4/3. Then multiply by 1/4 since you are guessing and there are 4 possible choice (a, b, c, d). (4/3)*(1/4)=(1/3), or 33%. Remember we cannot know that two answers are the same before guessing, because then it wouldn't be random.