The type of radioactive decay in which the mass number and atomic number remain unchanged is a) beta radiation. b) alpha radiation. c) positron radiation. d) gamma radiation.

Beta radiation is when a neutron decays into a proton and high speed electron, which would increase the atomic number by 1.
Alpha radiation is the emission of two neutrons and two protons (helium nucleus) which decreases both the mass and atomic numbers.
Positron emission is a proton becoming a neutron, emitting a positively charged electron and thus reduces the atomic number and increases the mass number.
Therefore the only remaining option is D, gamma, which does not involve the emission of any fermions but instead is a type of electromagnetic radiation.
Basically, it's D.