If it was totally random, surely it's none of the below? 50% chance of 25%, 25% chance of 50% and 25% chance of 60%.
Just to add confusion; where is the answer set limited. Maybe if it said: If you choose an answer to this question, from the responses provided below, at random, what is the chance you will be correct...........then you could use the defined set of answers.
In this case you could surmise that the answer is from the included list, but that doesn't mean it actually is. So from my point of view the set hasn't been defined and an answer of 3 squirrels is just as appropriate as an answer of a, b, c, or d.