c
(c) \(\frac{1}{\lambda }=R\,\left( \frac{1}{{{2}^{2}}}-\frac{1}{{{4}^{2}}} \right)=\frac{3R}{16}\Rightarrow \lambda =\frac{16}{3R}=\frac{16}{3}\times {{10}^{-5}}cm\)
Frequency \(n=\frac{c}{\lambda }=\frac{3\times {{10}^{10}}}{\frac{16}{3}\times {{10}^{-5}}}=\frac{9}{16}\times {{10}^{15}}Hz\)