Which of the following describes a nuclear reactor?
a nuclear reactor converts heat energy into nuclear fusion energy.
a nuclear reactor converts energy from fission nuclear energy into heat energy.
a nuclear reactor collides two nuclei together at high speeds to produce fusion nuclear energy.
a nuclear reactor converts fission nuclear energy into fusion nuclear energy.

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.