How many daughters does M have ?
a: P and Q are the only daughters of N.
b: D is brother of P and son of M. D and Q are siblings.

Using statement a, we know that P and Q are only daughters of N. But we cannot establish any relationship between N and M. Hence, statement a alone is insufficient.

Using statement b, we know that D,Q and P are the children of M. Bot we do not know whether Q is a male or female member. Hence, statement b alone is insufficient.

Using both statements A and B,we can conclude that D,P and Q are siblings and they are children of M and N. Also, it is clear that P and Q are the only daughters of M and N (from statement a). Hence, both the statements together are necessary to solve the question. Option E is the right answer.