In such questions , we have to understand the significance of mod value of a counter and difference between the operations of various types of counters , the nature of waveforms being produced by them etc.So I mention the steps one by one :
a) A ring counter is a type of counter composed of a type of circular shift register. The output of the last shift register is fed to the input of the first register.So in this periodicity comes out to be simply n cycles for n bit counter means value is going to be repeated every n cycles.So the frequency of the clock which is given as input is going to be divided by the same mod value which is the period of repetition when it is fed to a counter.
So in this case we have 25 bit Ring Counter.So mod value = 25
Therefore , frequency resulted due to ring counter = Input frequency / mod value
= 2 * 106 Hz / 25
= 80000 Hz
b) Now this output signal is fed to ripple counter .Now the characteristic of ripple counter is it is purely asynchronous counter and hence if we have say n flip flops , then the clock frequency of ith flip flop is provided by (i-1)th flip flop.So we have mod value of n bit ripple counter = 2n .Here we have 4 bit ripple counter .So mod value = 24 = 16
Therefore , frequency resulted due to ripple counter = Frequency provided to it / Mod value
= 80000 / 16
= 5000 Hz
Now this is fed to Mod 25 counter.
c) As now we have Mod 25 counter , so ,
frequency resulted due to this counter = Frequency provided to it / Mod value
= 5000 / 25
= 200 Hz
d) Now we have 4 bit Johnson's Counter .It's mod value is 2n for n bit counter as its operation is slightly different from ring counter but it is also prepared using shift register.So for 4 bit Johnson's counter , mod value = 2 * 4 = 8.Hence ,
frequency resulted due to this counter = Frequency provided to it / Mod value
= 200 / 8
= 25 Hz
Hence the final output frequency is 25 Hz