a b c d

d

$\text{Frequency } \nu = K(\frac{1}{(n-1)^2} - \frac{1}{n^2})= \frac{k(2n-1)}{(n-1)^2 n^2} = \frac{2k}{n^3}$ $\Rightarrow \nu \propto \frac{1}{n^3}$