In a hydrogen like atom electron makes transition from an energy level

Question:

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

1. $\frac{1}{n}$

2. $\frac{1}{\mathrm{n}^{2}}$

3. $\frac{1}{n^{3 / 2}}$

4. $\frac{1}{n^{3}}$

Correct Option: , 4

Solution: