In helium (He⁺), an electron transits from the 3rd excited state to the 1st excited state and emits a photon having energy e. What transition of an electron in Li²⁺ emits a photon of energy e?
a) From n=4 to n=2
b) From n=2 to n=1
c) From n=3 to n=1
d) From n=5 to n=2