In a hydrogen like atom electron makes transition from an energy level with quantum number $n$ to another with quantum number $(n − 1).$ If $n >> 1$, the frequency of radiation emitted is proportional to :

Question

$\begin{array}{1 1} (1) \large\frac{1}{n} \\ (2)\large\frac{1}{n^2} \\ (3) \large\frac{1}{n^{3/2}} \\ (4) \large\frac{1}{n^3} \end {array}$