Answer: c. gamma ray
Explanation: There are various process in which a radioactive nuclei decays:
1) Alpha Decay: In this process, a heavier nuclei decays into lighter nuclei by releasing alpha particle. The mass number is reduced by 4 units.
A= mass number
Z= atomic number
2) Beta particle: It is a type of decay process, in which a proton gets converted to neutron and an electron neutrino. This is also known as
-decay. In this the mass number remains same.

3) Gamma ray emission: It is a decay process in which an unstable nuclei gives excess energy by a spontaneous electromagnetic process. This decay releases
. This process does not change the mass number.
