Answer:
Option (A)
Step-by-step explanation:
Radioactive decay is usually defined as a process by which an unstable nucleus of an atom continuously disintegrates into a stable nucleus, resulting in the liberation of a large amount of energy as well as matter.
The radioactive substances undergoes radioactive decay and these released energy is very harmful as they are emitted in the form of ionizing radiation.
These process of radioactive decay is commonly known as radioactivity.
Thus, the correct answer is option (A).