Final answer:
The energy of a photon emitted by an atom transitioning from an excited state to a lower energy level is equivalent to the energy difference between those two states.
Step-by-step explanation:
When an atom in an excited state emits a photon of radiation, the energy of the photon is equal to the difference in energy between the atom's excited state and its final state. This is consistent with the law of conservation of energy, which states that energy cannot be created or destroyed. The photon released has an energy that corresponds to the specific energy lost by the atom as it transitions from a higher energy level to a lower one. Therefore, the correct answer to the student's question is d. the difference in energy between the atom's excited state and its final state.