Answer: Option A. actinides
Explanation:
Radioactive element is an element whose nucleus degenerate spontaneously due to the emission of alpha and beta particles, or gamma rays.
Radioactive elements are majorly comprise of Actinides.
Actinides also called actinoids, all are radioactive elements and release energy on radioactive decay. Uranium and thorium are naturally occurring whereas plutonium are synthetic actinides which are most abundant actinides on Earth.
Hence, the correct option is A.
