Ionisation potential of hydrogen atom is 13.6 eV. When hydrogen atoms in ground state are excited by a supply of 12.1 eV, then the number of spectral lines emitted by hydrogen atoms according to Bohr’s theory is..

So, the energy of electron in excited state $=-13.6+12.1=-1.5\mathrm{eV}$

$\text{ ie, }{E}_n=-\frac{13.6}{n^2}\mathrm{eV}$

$\begin{array}{r}-1.5=\frac{-13.6}{{\mathrm{n}}^2}\\ \Rightarrow {\mathrm{n}}^2=\frac{-13.6}{-1.5}\approx 9\\ \therefore \mathrm{n}=3\end{array}$

ie, energy of electron in excited state corresponds to third orbit.

The possible spectral line are when electron jumps from orbit 3rd to 2nd; 3rd to 1st and 2nd to 1st. Thus, 3 spectral lines are emitted.