The Bohr Theory Explains That an Emission Spectral Line Is

The Bohr theory explains that an emission spectral line is:

A) due to an electron losing energy but keeping the same values of its four quantum numbers
B) due to an electron losing energy and changing shells
C) due to an interaction between electrons in two different principal shells
D) due to an electron gaining energy and changing shells
E) due to an increase in the principal quantum number n of an electron