Which of the following best describes nuclear fission?

Nuclear fission is a process where the nucleus of a heavy atom, like uranium or plutonium, splits into two smaller nuclei, along with the release of energy and additional neutrons. This splitting occurs when a nucleus absorbs a neutron, becomes unstable, and divides. The formation of two or three neutrons during the fission reaction is significant because these neutrons can further strike other nuclei, leading to a chain reaction, which is the basis for both nuclear reactors and atomic bombs.

The other options describe processes related to nuclear interactions but do not accurately define fission. For example, the first option refers to fusion, which is the combining of light nuclei rather than their splitting. The third option incorrectly states that fission results in the formation of helium; that is a characteristic of fusion reactions. Lastly, extreme pressure is a condition associated with nuclear fusion, not fission, where lighter elements join together to form heavier elements. Understanding these distinctions highlights why option B is the most accurate representation of nuclear fission.