the question is for asynchronous circuit(ripple counters) the output of the flip flop is given as input to the next flip flop.
here the diagram is reversed to be asynchronous circuit
hence the lsb is Q0.
asking for Q3, which is 4th flip flop.
1 flip flop divides the frequency by 2.
thus, frequency of the waveform of Q3 = 1MHz / 4 = 62.5 KHz