What Quantum Transition Occurs Inside a Hydrogen Atom to Produce

Multiple Choice

What quantum transition occurs inside a hydrogen atom to produce a 21-cm radio photon?

A) An electron falls from the level n = 100 to the level n = 99 in the atom.
B) An electron in the ground atomic state reverses its direction of spin with respect to that of the proton.
C) An electron reverses the direction of its motion in orbit around the proton.
D) The electron combines with the proton in the nucleus to become a neutron, producing energy.