A free-moving neutron is absorbed by the nucleus of an atom. This can cause:
A. the protons in the nucleus to begin to search for more protons.
B. the atom to become unstable and rip apart.
C. the bonds in the nucleus to become stronger and release heat.
O D. the atom to absorb more neutrons to balance the protons.

the atom to become unstable and rip apart.

Explanation:

We define radioactivity as the spontaneous disintegration of atoms. It could occur naturally or artificially.

Artificial radioactivity occurs either by the process of nuclear fusion (combination of two light nuclei) or nuclear fission(bombarding of a heavy nucleus with a small particle such as a neutron).

When a heavy nucleus is bombarded with a neutron, the nucleus becomes unstable and rips apart producing a daughter nucleus/nuclei and more neutrons hence the process continues. This is otherwise called a chain reaction.