The correct option is B The energy needed to split the nucleus into its parts
It is observed that mass of a stable nucleus is always less than the total mass of constituent nucleons.This difference of mass is known as mass defect. When a nucleus is formed from the free nucleons mass defect is released in the form of energy by Einstein's mass-energy relation. This energy is used to bind the nucleons to form a nucleus therefore an equivalent amount of energy is required to split the nucleus into its parts, that is called binding energy of nucleus.