The correct answer is B) reverse power relay. A reverse power relay is used to prevent an AC generator from becoming motorized. When the generator experiences a reverse power condition, where the load is driving the generator instead of the generator driving the load, the reverse power relay will detect this and automatically disconnect the generator from the system. This prevents the generator from being driven as a motor, which could damage the generator and the prime mover. The other answer choices are not directly related to preventing a generator from becoming motorized. Overspeed trips, governor controls, and back pressure trips are used for other purposes, such as protecting the generator from overspeed or preventing damage to the prime mover, but they do not specifically address the issue of reverse power.