What energy process involves fission or fusion reactions?

Nuclear power is associated with the energy processes of fission and fusion reactions, making it the correct choice. In fission, heavy atomic nuclei such as uranium-235 or plutonium-239 are split into smaller nuclei, releasing a substantial amount of energy. This process is commonly used in nuclear reactors to generate electricity.

In contrast, fusion involves combining light atomic nuclei, such as hydrogen isotopes, to form a heavier nucleus, which also releases energy. This is the process that powers the sun and other stars. Although fusion is not yet a practical energy source for electricity generation on Earth, it holds promise for the future.

The other options are distinct forms of energy generation. Solar energy harnesses energy from the sun's rays through photovoltaic cells or solar thermal systems. Hydroelectric power converts the energy of flowing water into electricity, primarily using dams. Wind energy captures kinetic energy from wind through turbines. None of these methods involve fission or fusion reactions, which is why they do not correctly answer the question.