This is a conditional probability problem.
P(A|B) = P(A and B) / P(B)
P(A and B) = prob of one of the cards is 5 and sum is 8
5,3 or 3,5 , prob = 2/36 = 1/18
P(B) = prob of sum of 8 = 5/36
now required answer = (1/18)/(5/36) = D
Good call. I didn't realize that it had to be conditional and I was stuck there wondering why I kept getting 2/36.