Final answer:
Nuclear fission continues on its own because each fission event creates additional neutrons that cause further fissions, leading to a self-sustaining chain reaction. So the correcct option is d.
Step-by-step explanation:
Once started, nuclear fission can continue to occur on its own because each fission event typically produces more free neutrons. The correct answer to the question is (d) A neutron begins the reaction, and more neutrons are formed in it. These additional neutrons can then go on to induce fission in other nuclei, which in turn release more neutrons. This process can result in an exponential increase in the number of fissions, such as 1 → 2 → 4 → 8 → and so on, which is known as a chain reaction. Self-sustained fission is possible because neutron-induced fission also produces neutrons that can invoke additional fissions, provided that there is a minimum critical mass present to achieve criticality. This critical mass ensures that enough neutrons are available to sustain the reaction. If the number of neutrons produced is sufficient to induce at least one more fission, the chain reaction will become self-sustaining, releasing significant amounts of energy.