Answer: The correct answer is its chain reactions can be initiated by neutrons.
Explanation: There are 2 types of nuclear reactions:
1. Nuclear fission reaction: In these type of reaction, one larger nuclei breaks down into two or more smaller nuclie's. Its chain reaction is initiated when the nucleus is struck with neutrons.
2. Nuclear fusion reactions: In these type of reaction, two small nuclei combine together to form a single large molecule. This type of reaction requires high temperature to occur.
Both these reactions occur due to the change in the nuclei of an atom.
From the above information, the nuclear fission chain reactions re initiated by the neutrons.