What Quantum Transition Occurs Inside a Cool Hydrogen Atom to Produce

Multiple Choice

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

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