The correct answer to the question is Fusion i.e the process of combining two small nuclei into one nucleus of larger mass is called fusion.
EXPLANATION:
Before answering this question, first we have to understand nuclear fission and nuclear fusion.
Nuclear fission is the thermo nuclear process in which a heavy unstable nucleus will split into two light stable nuclei with the release of energy.
For instance, the splitting of Uranium-235 to Krypton-92 and Barium-141.
Nuclear fusion is just the opposite process of nuclear fission. It is the thermo nuclear process in which two light unstable nuclei will combine with each other at a high temperature equal to the temperature of sun to form a large stable nuclei with the release of energy.
For instance, we may take hydrogen bomb i.e the energy formation in stars. Here, four hydrogen nuclei will combine with each other form a helium nuclei with the release of huge amount of energy.
Hence, the correct answer to the question is nuclear fusion.
