You're randomly picking an answer out of 4 possible answers, so that 1/4 chance, or 25%. However, "25%" which is correct, appears twice as two possible answers, so that's 2/4, or 50% chance of randomly picking the right answer.
So the answer is 50%. NOT "B.) 50%", just 50%.
It appears you need to answer this question by providing an answer, not by selecting one presented to you.