Which factor is needed to begin a nuclear fission reaction?
high temperature
stable nuclei
addition of a neutron
low pressure

addition of a neutron

Explanation:

Nuclear fission is a radioactive decay process in which a heavy nucleus spontaneously disintegrates into lighter ones accompanied by the release of energy.

An atom whose neutron/proton ratio is the same as that of the stability ratio of that atom is said to be stable. When such an atom is bombarded with a neutron particle, the stability ratio is offset. What results is a radioactive decay of such a nucleus. This form of decay is a nuclear fission.

A series of chain reaction is produced though this until the stability ratio is reached.