I'm assuming you're asked to find [tex]\mathbb P(A\mid B)[/tex], not [tex]\mathbb P\left(\dfrac AB\right)[/tex] (which is nonsense as far as I can tell).
By definition,
[tex]\mathbb P(A\mid B)=\dfrac{\mathbb P(A\cap B)}{\mathbb P(B)[/tex]
We're told the two events are mutually exclusive, which means their intersection is disjoint and the probability of their intersection is 0. So the answer is C.
