Gamma decay refers to the emission of gamma rays from a nucleus. Gamma rays are high-energy photons and do not carry any charge. They are typically emitted from the nucleus during radioactive decay when the nucleus transitions from a higher energy state to a lower energy state. Unlike alpha and beta decay, gamma decay does not involve the emission of particles such as protons or neutrons from the nucleus. Therefore, the emission of gamma rays does not change the composition of the nucleus in terms of its protons and neutrons. Given this understanding: Option A - Incorrect. This describes alpha decay, where the mass number is reduced by 4 and the atomic number is reduced by 2 , not gamma decay. Option B - Incorrect. This is more characteristic of beta decay, where a neutron is converted into a proton, increasing the atomic number by 1 , but the mass number remains unchanged. Again, not gamma decay. Option C - Correct. During gamma decay, neither the mass number nor the atomic number of the atom is changed. Gamma decay only involves a change in the energy state of the nucleus, with no alteration in the number of protons or neutrons. Option D - Incorrect. This also describes alpha decay, not gamma decay. Therefore, the correct answer is Option C : Mass number and atomic number are not changed during gamma decay.