Let's consider each options
D)
K or 2^n bit JC will have 2^(n+1) states, but the decoder outputs 1 out of 2^n states of binary counter. So this option is clearly wrong
A,B)
Each of the counters here for k or 2^n bit, will have 2^(2^n) output, but decoder in the circuit has outputs 1 out of 2^n.
So, A,B are also wrong here
Now let's C) Ring counter
K or 2^n bit RC will have K or 2^n states, which reflects the given situation. It's the right option