We know that ripple counter is an asynchronous counter and hence the state waveform of Q1 is dependent on its previous flip flop i.e. corresponding to Q0 as the clock frequency of Q1 is provided by Q0 flip flop output.Hence the propogation delay of each of the 8 flips is going to be considered for calculating clock frequency.
Also the strobe signal is required to remove the problem of glitch or we can say decoding error.This occurs when counter has reached last value in this case 255(1111 1111 in binary) to 0 , so in between that intermediate transition occurs and hence decoder gives wrong values.To remove this ambiguity strobe signal is provided additionally , so this is also going to be added in overall propogation time of counter's operation.
So overall propogation delay = Delay of 8 flip flops + Strobe pulse time
= 75 * 8 + 50
= 650 ns
Therefore , clock frequency required for proper operation = 1/650 ns
= 1.5 MHz(approx)
Hence C) is the correct answer.