How is nuclear (radioactive) decay best defined?

Nuclear decay refers to the process by which an unstable atomic nucleus loses energy in the form of radiation. This loss of energy typically occurs during the transmutation of the nucleus into a more stable configuration. While it involves various forms of radiation, including alpha, beta, and gamma radiation, the fundamental aspect of this process is the release of energy tied to the instability of the nucleus.

An unstable nucleus inherently contains excess energy that it cannot maintain indefinitely. Through the decay process, the nucleus transitions towards a lower energy state by emitting particles or electromagnetic waves, thereby seeking stability. This aligns with the reason why the option that suggests "an unstable nucleus releasing energy" is the best definition of nuclear decay.

In contrast, the other options either mischaracterize the process or describe unrelated phenomena. For instance, an unstable nucleus absorbing energy does not constitute decay but rather an excitation process. Moreover, fusion refers to the combining of lighter nuclei to form a heavier one, which is distinct from decay. Lastly, while the process of decay ultimately leads to an unstable atom becoming stable, it is the released energy during this transition that fundamentally defines nuclear decay, making the option that captures energy release the most accurate definition.