The answer is D).
This is a worked solution if found:
There are only 4 different blood groups: A, B, AB and O.
The possibility of the criminal having blood group AB is: 25% (one out of the four possible blood groups).
However i disagree, the question seems to be asking P(AB knowing one allele is B) not P(AB) in general?
How would you interpret the phrasing of the question?