Final answer:
After being excited by absorbing a photon, an atom returns to its ground state by emitting a single photon of light corresponding to the energy difference between the excited and ground states.
Step-by-step explanation:
When an atom in the ground state absorbs a single photon of light, it becomes excited and moves to a higher energy level. This process is called excitation. Once an atom is excited, it will not remain in this state for very long. Typically, in about a hundred-millionth of a second, it will relax back to the ground state. As it relaxes back, it emits a single photon of light, which corresponds to the energy difference between the excited state and the ground state. This emission of light can occur in one jump, or it may happen in steps through intermediate levels—the emitted light's wavelength will match the energy transition between the respective levels.
In summary, after an atom relaxes back to its ground state upon excitation, the correct answer is: 1) It emits a single photon of light.