Which of the following reactions is caused by the bombardment of a nucleus by atomic particles or other nuclei?

The correct choice describes a process in which a nucleus is struck by atomic particles or other nuclei, leading to various nuclear reactions. This can occur when one or more particles, such as neutrons, protons, or alpha particles, collide with a target nucleus, resulting in transformations that can produce new elements or isotopes.

Nuclear fission involves the splitting of a heavy nucleus into two lighter nuclei, accompanied by the release of energy, and while it can be initiated by bombardment, it is not exclusively defined as such.

Nuclear fusion refers to the process where two light atomic nuclei combine to form a heavier nucleus, typically occurring under extreme conditions, such as in stars. Fusion does involve interaction, but it does not conform specifically to the definition in the question.

A chain reaction involves a series of reactions where the products of one reaction initiate subsequent reactions; this is often seen in fission processes but does not specifically define the act of bombardment by particles or nuclei.

Thus, the concept of nuclear bombardment reactions specifically emphasizes the interaction initiated by the bombardment of a nucleus, aligning precisely with the description provided in the question.