The correct option is A Sommerfeld
Sommerfeld introduced the concept that electrons travel in elliptical orbitals. Bohr was not able to explain the reason for the fine spectral lines visible by high revolving power spectroscopes but Sommerfeld explained the reason for the same. He said that the energy of the stationary orbit depends not only on n but on k to some extent as well. So, when a transition of electron from a higher level to a lower level occurs.