@slh C 50%.. I would rule out B right away leaving only 25% and 50% as potential answers, to satisfy the question and pick at random I would now randomly pick one of those. Since there are only two choices, 25% or 50% i know that only 50% must be correct of these two, and thus 50% is the answer.

We can likewise know that is the only answer because I as far as I can tell there is no other scenario, no matter how convoluted (and that was pretty damn convoluted) that would give an answer :)