There’s no joke; the 4th choice (in addition to 25%, 25%, 50%) is arbitrary; it could be 0%, 100%, or anything other than 25% or 50%. The only purpose it serves is to be a 4th option (thus making the probability of choosing any individual answer 25% when choosing randomly).
The question would work just as well if it had 3 options:
There’s no joke; the 4th choice (in addition to 25%, 25%, 50%) is arbitrary; it could be 0%, 100%, or anything other than 25% or 50%. The only purpose it serves is to be a 4th option (thus making the probability of choosing any individual answer 25% when choosing randomly).
The question would work just as well if it had 3 options:
1/3, 1/3, 2/3.
I think having 0% as the 4th choice would have actually made it more interesting.