The number of spectral lines that are possible when electrons in fourth energy level in different hydrogen atoms return to ground state.

The correct option is C 6
When electron transition from $n^{th}$ to ground state then $\Sigma (n-1)$ spectral lines are produced

fourth energy level means n = 4

Thus if electrons transiton from fourth energy level then $\Sigma (4-1)=3+2+1=6$ spectral lines are produced

Hence, the correct option is $\text{C}$