Answer:
A nuclear reactor converts energy from fission nuclear energy into heat energy.
Step-by-step explanation:
A nuclear reactor is a device used to manage nuclear reactions. Basically, it consist in making collide to nuclei or other subatomic particle, to produce a controlled nuclear fission, which released energy that it's converted in thermal energy, which it's also converted for a final purpose like mechanical movements. Remember that nuclear fission refers to decay of an atom, its separation into particles.
So, based on these concepts, the right answer is A nuclear reactor converts energy from fission nuclear energy into heat energy. Because other option are talking about nuclear fusion, which is not involved in a nuclear reactor.