When electrons in sample of H-atom make transitions from 5th excited state to ground state, then the maximum number of spectral lines which belong to visible region is:

The correct option is A $10$

Given,

Electron is in $5^{th}$ excited state,

$\Rightarrow n_2=6$

Electron dexcite to visible region,

$\Rightarrow n_1=2$

Now,

Number of spectral lines $=\frac{(n_2-n_1)(n_2-n_1+1)}{2}$

$=\frac{(6-2)(6-2+1)}{2}$

$=\frac{4\times 5}{2}$

$=10$

Hence, option A is correct.