The number of spectral lines that are possible when electrons in $7$th shell in different hydrogen atoms return to the $2$nd shell is:

The correct option is C $15$

The general formula for calculating number of spectral lines

$=\frac{(n_2-n_1)(n_2-n_1+1)}{2}$

Here $n_2=7,n_1=2$

Number of spiral lines $=\frac{(7-1)(7-2+1)}{2}$

$=\frac{\left(5\right)\left(6\right)}{2}$

$=15$