Answer. C
Key lies in noting that there is a 90° phase lag -
1. between f and output of FF1 if it is positive edge triggered, since f changes for every negative edge.
2. between the two FFs as one is positive edge triggered and the other is negative edge triggered.
So,
A. FF1 output = ((f')') = f and FF2 output = f$\angle$ 90°
B. FF1 output = f'$\angle$ 90° and FF2 output = f'$\angle$ 180° = f (since f is a periodic digital signal with 50% duty cycle)
C. FF1 output = f$\angle$ 90° and FF2 output = f$\angle$ 180°
D. FF1 output = f' and FF2 output = f'$\angle$ 90°
This explains the timing diagrams and @Arjun sir's answer.