The correct answer is
A. Chain reaction
This is how the chain reaction works in a process of nuclear fission:
1) Slow-moving neutrons reach heavy, unstable, radioactive nuclei (such as Uranium-235)
2) The neutrons interact with the nuclei of Uranium, which break apart producing smaller nuclei and additional neutrons
3) The neutrons produced in the reaction are slowed down by a moderator (e.g. water) and they interact with new nuclei of Uranium, causing new fission reactions)
4) The new neutrons interact with new nuclei of Uranium, and so on...
