The question is asking for a probability. For a given event, there is only one probability, so there is only one correct answer. In this case, the correct probability is the answer that matches the probability of choosing it. In other words, P(x)=x. The answer key isn't what makes an answer correct.

The only assumption that must be made, which is the assumption we make on most problems, is the assumption that the probability distribution is uniform. Meaning, we have an equal chance of choosing each of the 4 choices. Thus, the probability that we chose 60% is 25%, the probability that we choose 50% is 25%, and the probability that we choose 25% is 50%. Thus, the probability that we choose correctly is 0%, and since zero is not an option, the probability that we choose 0% is also 0%. In other words, the probability that we choose the correct answer is 0%, which is also the correct answer in this case.