Find the equation of Next State, Qn = D (from D F-F) = Q.X' + Q'.Y (here Q is previous state)
If we draw the characteristics table of Y X Q to find Qn using the above equation:
Y X Q Qn
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 0
(YX->Qn) : (00-latch mode, 01-reset mode, 10-set mode, 11-toggle mode)
So, It is J K Flip-flop with J=Y and K=X.
Answer is D.