a b c d

c

$ \Delta E = h\nu = E_n - E_{n+1} = E_0 (\frac{1}{n^2} - \frac{1}{(n+1)^2}) = E_0 \frac{2n+1}{n^2(n+1)^2} $ Since n is large $\Rightarrow \nu \propto \frac{1}{n^3}$