I think, the chance to be right, or not, is always 50%. Because it is binary. You can be right, or not - there are no possible values between.

And 50% is one of the possible answers (B). So it is possible to finde the correct answer, which is 50%.
If the answer B (50%) would not exist, the chance to be correct will be 0%. But it exists, so the chance is 50%.