Answer: Option (A) is the correct answer.
Step-by-step explanation:
When a large atomic nuclei splits into two small atomic nuclei along with release of energy then this process is known as nuclear fission reaction.
For example,
A chain reaction is defined as the reaction in which products formed themselves lead to continuation of reaction again and again.
Hence, the neutrons released in the nuclear fission reaction again and again bombard with the uranium atom. Therefore, a chain reaction will be initiated.
Thus, we can conclude that the term that is used to describe a key factor required to start a fission reaction is chain reaction.