Answer:
It's A; fission.
Explanation:
Atomic reactions happen when atomic nuclei are split or joined together. There are two types of atomic reactions: fission and fusion. Nuclear fission is the splitting of an atomic nucleus to release its energy. In a fission reaction, a free neutron is smashed into an atom of U-235. The speeding neutron joins the nucleus and increases the atom's mass. The nucleus becomes U-236 and now has 144 neutrons. Uranium atoms don't "like" having 144 neutrons. The U-236 mass is unstable. It breaks apart into other elements. The breaking U-236 releases several neutrons and much energy. The released neutrons soon smash into other atoms of U-235. The same process recurs, and a chain reaction occurs.
Great amounts of energy are released from a chain reaction. This reaction is used as the basis for the atomic bomb and nuclear power plants.
