The correct answer is C) reduce high turbine RPM to an efficient propeller RPM. The main reduction gears in a ship's propulsion system are used to convert the high rotational speed of the engine or turbine to a lower, more efficient speed for the propeller. Propellers operate most efficiently at lower rotational speeds, so the reduction gears allow the high-speed engine or turbine to be matched to the optimal propeller speed. This improves the overall efficiency and performance of the propulsion system. The other answer choices are incorrect because: A) Noise reduction is not the primary purpose of the reduction gears, B) The reduction gears do not directly transmit vibration and thrust, and D) Providing a means of engine reversal is a separate function, not the purpose of the reduction gears.