No. of possibilities for start state = 2 as it is fixed here.
No. of possibilities for final state = 2^2 = 4 as any subset of the set of states can be the final state.
No. of possible transition functions = Number of possible functions from a set of 4 elements (Q×Σ) to a set of 2 elements (Q)
=2^4 = 16
So, number of possible DFA's= 16*4 = 64
