how does alpha decay change the nucleus of radioactive atom.

When atom undergoes the alpha emission the original atom convert into the atom having mass number less than 4  and atomic number 2 less than the starting atom.

Explanation:

Alpha radiations are emitted as a result of radioactive decay. The atom emit the alpha particles consist of two proton and two neutrons. Which is also called helium nuclei. When atom undergoes the alpha emission the original atom convert into the atom having mass number less than 4  and atomic number 2 less than the starting atom.

Properties of alpha radiation:

Alpha radiations can travel in a short distance.

These radiations can not penetrate into the skin or clothes.

These radiations can be harmful for the human if these are inhaled.

These radiations can be stopped by a piece of paper.

₉₂U²³⁸   →   ₉₀Th²³⁴  + ₂He⁴  + energy

the answer would be c