The best and most correct answer among the choices provided by your question is the first choice which is "<span>An unstable nucleus decays into a more stable nucleus."
</span>Radioactive decay<span> (also known as </span>nuclear decay<span> or </span>radioactivity<span>) is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays, and conversion electrons. A material that spontaneously emits such radiation is considered </span>radioactive<span>.</span>
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