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In contrast to the other responses, I maintain that the correct answer is (A), though I would say that it is poorly expressed.
A nuclear reactor is designed to operate at a constant power, and is not designed to explode. That means that, on average, each fission event in the reactor leads ultimately to exactly one more fission event, and so on into the future. If a fission event led to more than one fission event, the power generated would increase exponentially. If it led to less than one fission event, the power would decay exponentially. So, a reactor is designed so that if the power goes too high, the fission reaction becomes less efficient (it loses more neutrons to the surroundings). This means that a reactor is in a natural negative feedback cycle. The fission rate can increase, but if it does, it reduces the neutron efficiency of the reactor, and so the fission rate cannot increase without bound. I believe that the opposite direction, too little fission, is generally corrected by active means, retracting control rods or something similar. Usually several physical means are used to assert the feedback that prevents the reactor from producing too much power. In a CANDU reactor, hot fuel rods expand and become less efficient at absorbing neutrons. At the same time, the heavy water moderator becomes less dense, and slows down fewer of the neutrons to the low speeds necessary for the fission to continue. So, purely physical effects prevent the reactor from generating power above its design rating.
(B) is false, both reactors and bombs make use of chain reactions.
(C) is false, nuclear bombs do not make use of controlled fission reactions.
(D) is false, because if a reactor has less than critical mass, it cannot produce power. Similarly, a bomb at less than critical mass does not explode.
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