Hydrogen atom in ground state is excited by a monochromatic radiation of $\lambda$= 975 $\stackrel{o}{A}$. Number of spectral lines in the resulting spectrum emitted will be 

 

If excitation wavelength is $975\text{Å}$
then its energy, in ev, is $\frac{12400}{975}=12.71\mathrm{eV}$
Now electron energy in H atoms are again
$\begin{array}{r}13.6\left(\frac{1}{1}-\frac{1}{{\mathrm{n}}^2}\right)=12.71\\ 1-\frac{1}{{\mathrm{n}}^2}=\frac{12.71}{13.6}\Rightarrow \mathrm{n}=4\end{array}$
If n = 4,
no. of spectral emissions possible,
$\frac{\mathrm{n}(\mathrm{n}-1)}{2}=\frac{4\times 3}{2}=6$