Step-by-step explanation:
A nuclear fission corresponds to the partition of a heavy and unstable atomic nucleus, giving rise to two medium atomic nuclei. This division of the nucleus is done through the bombardment of moderate neutrons that have no electrical charge and can penetrate the nucleus without being repelled. In addition to the new atomic nuclei that are produced, fission also releases neutrons and an enormous amount of energy.
Uranium is often used as reactant in a nuclear fission reactor.
Answer: b. Uranium.