#### Answer

The correct answer is: (e) 6

#### Work Step by Step

When an electron drops down from a higher energy level to a lower energy level, it emits a photon with a specific wavelength. The photons emitted by hydrogen atoms would show up as emission lines at specific wavelengths.
The following emission lines are possible when the electron is at n=4:
from n = 4 to n = 3
from n = 4 to n = 2
from n = 4 to n = 1
from n = 3 to n = 2
from n = 3 to n = 1
from n = 2 to n = 1
The correct answer is: (e) 6