Which best explains what happens when an electron moves from an excited state to the ground state?

An electron has a higher amount of energy in its excited than it does in its ground state. In order to make this transition downwards, the electron must release some of this energy that it possesses. This energy is released in the form of a photon, a packed of energy. These photons have energy equivalent to the difference in energy of the electron in its excited state and ground state.