Qn is stable over which of the following clock regions?
3 is CORRECT. During the nth clock pulse, a flip-flop is awake and may change its output. When the clock goes down low (Region C), the flip-flop "goes to sleep" and remembers the output value attained while the CLOCK was high (Region B), namely Qn. Thus Qn is stable when the CLOCK is low after the nth clock pulse.