The maximum number of emission lines when the excited electron of a H atom in n = 6 drops to the ground state is .

The correct option is A 15
The number of lines produced when an electron from the $n^{th}$ shell drops to the ground state is $\frac{n(n-1)}{2}$

So the required number of emission lines is $\frac{6(6-1)}{2}=15$