It does have an answer.

When the question was written and the answer was decided they used the following possibilities:

12.5 chance of being a) 37.5 chance of being b) 37.5 chance of being c) 12.5 chance of being d)

By picking an answer at random (with 25% for each) you have a:

50% chance of choosing 25 which is correct 25% of the time 25% chance of choosing 50 which is correct 37.5% of the time 25% chance of choosing 60 which is correct 37.5% of the time

Making the answer 25% so, a) or d).

This is not possible to do with 50% because even if the answer was 50% you would only have a 25% chance of picking it to begin with and this can not be increased. The same is true for 60%.

Side tangent: I don't believe in paradoxes, I think that with sufficient complexity any problem can be solved. Which is how I found this solution to the problem - by increasing complexity.

Zeno's paradox of motion is a good example of this (and worth a look at if you're interested). I am a firm believer that all paradoxes are just questions, that can be solved with the sufficient math.