The correct answer is D) It transfers high-speed gas turbine rotation to low-speed propeller rotation. The main reduction gear is responsible for reducing the high rotational speed of the gas turbine to the lower rotational speed required by the ship's propeller. This is necessary because gas turbines typically operate at much higher speeds compared to propeller shafts. The reduction gear allows the efficient transfer of power from the high-speed turbine to the low-speed propeller. The other answer choices are incorrect because they do not accurately describe the primary function of the main reduction gear, which is to reduce rotational speed rather than increase it.