When the excited electron of a H atom from n = 5 drops to the...

Maximum number of emission lines

$= \dfrac{\Delta n\left(\Delta n + 1\right)}{2}\Delta n = {n}_{2} - {n}_{1}{n}_{2} = 5{n}_{1} = 1$

Maximum number of emission lines=$\dfrac{4\left(4 + 1\right)}{2} = \dfrac{20}{2} = 10$

These lines correspond to the lyman series of the hydrogen spectrum