What Happens When the Electron in a Hydrogen Atom Flips $\alpha$

Multiple Choice

What happens when the electron in a hydrogen atom flips its direction of spin from parallel to antiparallel to that of the proton?

A) Nothing. This event is a forbidden transition that never occurs.
B) The atom emits a photon of 656.3-nm wavelength (H _$\alpha$ ) in the red region of the spectrum.
C) The atom emits a photon of 21-cm wavelength in the radio region of the spectrum.
D) The atom emits a photon of 121.5-nm wavelength (L _$\alpha$ ) in the UV region of the spectrum.