Gamma Decay

Gamma decay, as NCERT explains, occurs when a nucleus in an excited energy state relaxes to a lower state by emitting a high-energy photon called a gamma ray. Because a photon carries no charge and no rest mass, neither the atomic number nor the mass number of the nucleus changes; only its internal energy decreases. The option that atomic number increases describes beta-minus decay, not gamma emission, so it is wrong. The option that mass number decreases by four describes alpha decay and is unrelated to photon emission. The option that both numbers decrease combines features of alpha decay and is likewise wrong. As stated in NCERT Class 12, Chapter 13 (Nuclei), gamma emission usually follows alpha or beta decay because the daughter is frequently produced in an excited state, analogous to how atoms emit light when electrons drop between levels. A consistency check: since the emitted quantum is massless and chargeless, both conservation of charge and of nucleon number are trivially satisfied without altering $Z$ or $A$.