Answer: fusion
Explanation:
Nuclear fission : It is defined as a process which involves the conversion of a heavier nuclei (which is an unstable nuclei) into two or more small nuclei (which is a stable nuclei) along with release of high amounts of energy.
Example: The energy released from the process of nuclear fission which is converted into electrical energy that is used in our homes and factories.
Nuclear fusion : It is defined as a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.
Example: It occurs in sun and stars where the isotopes of hydrogen called as Tritium and Deuterium combine together to form a neutron and a helium atom.
