Which of the following describes the opposite of fusion?

The correct response is fission, as it specifically refers to the process in which the nucleus of an atom splits into two or more smaller nuclei, along with the release of energy. This process is essentially the opposite of fusion, which involves combining lighter atomic nuclei to form a heavier nucleus, also releasing energy.

Fission is commonly understood in the context of nuclear reactors and weapons, where the splitting of heavy nuclei (like Uranium-235 or Plutonium-239) is harnessed for energy. This contrasts sharply with fusion, which powers stars, including our sun, where hydrogen nuclei combine to form helium under extreme heat and pressure, resulting in tremendous energy output.

Other options, such as combination and condensation, do not directly relate to the nuclear processes of fusion and fission. Combination involves the joining of two or more substances, while condensation refers to the phase change from a gas to a liquid, neither of which captures the atomic-level changes seen in nuclear reactions. Separation is a more general term that could refer to breaking mixtures or compounds apart but doesn't accurately describe the specific nuclear process opposite to fusion.