Answer: Option (D) is the correct answer.
Explanation:
Nuclear chain reactions occur in fission reactions only. Nuclear fission reactions are defined as the reactions in which larger nuclide splits into two or more smaller daughter nuclides.
For example, [tex]^{235}_{92}U + ^{1}_{0}n \rightarrow ^{90}_{36}Kr + ^{144}_{56}Ba + 3^{1}_{0}n[/tex]
These neutrons released will again bombard with the uranium atom leading to another fission reaction and this chain continues.
Thus, we can conclude that in a nuclear chain reaction, neutrons are used to trigger fission reactions.
