Answer: Option (a) is the correct answer.
Step-by-step explanation:
Nuclear fission is defined as a reaction in which a heavy nucleus splits into two or more small nuclei along with emission of energy.
For example,
So, the given diagram shows a large nucleus is splitting into two small nuclei. Therefore, it is a nuclear fission reaction.
Whereas a nuclear reaction in which two small nuclei combine together to result into the formation of a large nucleus is known as a nuclear fusion reaction.
Thus, we can conclude that the reaction depicted in the figure would take place in fission reactor.