c
Time period of revolution of electron in \(\mathrm{n}^{\text {th }}\) orbit \(\mathrm{T}=\frac{2 \pi \mathrm{r}}{\mathrm{V}}=\frac{2 \pi \mathrm{a}_{0}\left(\frac{\mathrm{n}^{2}}{\mathrm{Z}}\right)}{\mathrm{V}_{0}\left(\frac{\mathrm{Z}}{\mathrm{n}}\right)}\)

\(\Rightarrow \mathrm{T} \propto \frac{\mathrm{n}^{3}}{\mathrm{Z}^{2}}\)

\(\frac{\mathrm{T}_{2}}{\mathrm{T}_{1}}=\frac{(2)^{3}}{(1)^{3}}=8 \Rightarrow \mathrm{T}_{2}=8 \times 1.6 \times 10^{-16}\)

Now frequency \(f_{2}=\frac{1}{T_{2}}=\frac{10^{16}}{8 \times 1.6} \approx 7.8 \times 10^{14} \mathrm{Hz}\)