Answer : The correct option is, Radioactive.
Explanation :
Radioactive element : It is an element in which the nucleus breaks down and gives off energy and particles.
Radioactive decay : It the process in which an unstable atomic nucleus loses energy by emitting the radiations like, alpha particles, beta particles, or gamma rays.
The radioactive materials have unstable nuclei.
The naturally occurring radioactive elements are, radium, thorium, and uranium.
Thus, a radioactive element is one in which the nucleus breaks down and gives off particles and energy.