In a nuclear chain reaction, are used to trigger fission reactions.
a. electrons
b. protons
c. alpha particles
d. neutrons

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,

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.

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