A neutron begins the reaction and more neutrons are formed in it ( D )
When a nuclear fission is started it will continue to occur on its own because Nuclear fission is a form of reaction that starts with the splitting of the neutron of an atom into smaller nuclei which will continue to split into much smaller nuclei infinitely ( i.e. Chain reaction )
Hence we can say that once a Nuclear fission is started it can continue to occur on its own because A neutron begins the reaction and more neutrons are formed in it ( i.e. chain reaction )