Answer:
The` correct answer is radioactivity.
Explanation:
Fusion is the process by which two or more smaller nuclei combines together to form one bigger nucleus with release of an energy.
Meltdown is the process in which solid substance changes its state that is from solid to liquid.
Fission is the process ion which larger unstable nucleus break down into two or more smaller stable nuclei with release of of an energy.
Radioactivity is the process in which unstable atomic nucleus looses energy by emitting radiation like alpha particles, beta particles etc.
