d) the binding energy of the nucleus
The energy released in a nuclear reaction comes from the binding energy of the nucleus. The nucleus of an atom is held together by the strong nuclear force, which binds the protons and neutrons together. This binding force is associated with a certain amount of energy. When a nuclear reaction occurs, such as nuclear fission or fusion, the rearrangement of nucleons (protons and neutrons) leads to a change in the binding energy of the nucleus. The difference in binding energy between the initial and final states is released as energy. This released energy is what powers nuclear reactions and is often in the form of kinetic energy of particles, electromagnetic radiation, or both.