The correct answer is:
The longest wavelength photon is n = 5 to n = 4.
The explanation:
According to he relation between the wavelength and energy, there have inverse relationship. The longest wavelength has the lowest energy.
and when:
-Transitions to n=1 are in the UV part of the spectrum and have the shortest wavelengths.
-and Transitions to n=2 are in the visible part of the spectrum and have wavelengths between 700nm and around 400 nm
-and Transitions to n=3 or greater are in the infra-red part of the spectrum and have the longest wavelengths.
- So the longest wavelength would correspond to the lowest energy and that would correspond to the difference in energy between n=5 and n=4
