The correct answer is: D. Neither the mass number nor the atomic number of the nucleus changes.
Gamma decay is a type of radioactive decay in which a nucleus emits a gamma ray (a high-energy photon) without changing its atomic number or mass number. This type of decay is often seen in radioactive nuclei that are in an excited state. The gamma ray emitted carries away the excess energy, leaving the nucleus in a lower energy state.
Option A is incorrect because the mass number of the nucleus does not change in gamma decay. Option B is incorrect because the atomic number of the nucleus does not change in gamma decay. Option C is incorrect because both the mass number and the atomic number of the nucleus do not change in gamma decay.