Once started, why can nuclear fission continue to occur on its own?
a. the thermal energy released can cause the products to join.
b. the neutron formed by the reaction can cause more nuclei to combine.
c. the thermal energy released can cause the products to split.
d. a neutron begins the reaction, and more neutrons are formed in it.

D. A neutron begins the reaction, and more neutrons are formed in it.

Explanation:

The nuclear fission of uranium occurs when a nucleus of uranium-235 absorbs a slow-moving neutron, becoming a very unstable nucleus of uranium-236, which decays into several nuclei (for instance, krypton and barion) and emitting other neutrons (generally, three neutrons).

The neutrons emitted in the reaction are then slowed down by using a moderator, and then they are absorbed by other nuclei of uranium-235, inducing their fission in a chain-reaction process.