b
A hydrogen atom makes a transition from $n=2$ to $n=1$
Then wavelength
$=\operatorname{Rcz}^{2} \frac{1}{\mathrm{n}_{1}^{2}}-\frac{1}{\mathrm{n}_{2}^{2}}=\operatorname{Rc}(1)^{2}\left[1-\frac{1}{4}\right]$
$\lambda=\operatorname{Rc}\left[\frac{3}{4}\right]$ ..... $(1)$
Forionized lithium
$\lambda=\operatorname{Rc}(3)^{2}\left[\frac{1}{n^{2}}\right]=\operatorname{Rc} 9\left[\frac{1}{n^{2}}\right]$ ..... $(2)$
$\operatorname{Rc}\left[\frac{3}{4}\right]=\operatorname{Rc9}\left[\frac{1}{n^{2}}\right]$
$\Rightarrow \frac{3}{4}=\frac{9}{n^{2}} \Rightarrow n=\sqrt{12}=2 \sqrt{3}$
The least quantum number must be $4$.
