<u>Answer:</u> The correct answer is Option A.
<u>Explanation:</u>
For the given options:
Alpha decay is defined as the process in which alpha particle is emitted. In this process, a heavier nuclei decays into a lighter nuclei. The alpha particle released carries a mass of 4 units.

Beta decay is defined as the process in which beta particle is emitted. In this process, a neutron gets converted to a proton and an electron. The beta particle does not have any mass.

Gamma decay is defined as the process in which an unstable nuclei gives off excess energy by a spontaneous electromagnetic process and thus releases
. These radiations does not have any mass.

Nuclear reactions are defined as the reactions in which change of nucleus takes place.
Hence, the correct answer is Option A.