(N/A) The Equivalence of Mass and Energy: Albert Einstein proposed that mass and energy are interchangeable,related by the equation $E = mc^2$,where $E$ is energy,$m$ is mass,and $c$ is the speed of light in a vacuum $(3 \times 10^8 \ m/s)$. This implies that a small amount of mass can be converted into a large amount of energy.
$(b)$ Nuclear Energy: This is the energy released during nuclear reactions,such as nuclear fission (splitting of heavy nuclei) or nuclear fusion (combining of light nuclei). The energy released is due to the mass defect,where the mass of the products is slightly less than the mass of the reactants,with the difference converted into energy according to $E = \Delta mc^2$.
$(c)$ The Principle of Conservation of Energy: This principle states that energy can neither be created nor destroyed,only transformed from one form to another. In an isolated system,the total energy remains constant over time.