The correct answer is C) reduce high turbine RPM to an efficient propeller RPM. The main reduction gears on a vessel serve the purpose of transmitting power from the high-speed engine or turbine to the slower-turning propeller shaft. This reduction in rotational speed is necessary to match the optimal propeller RPM for efficient propulsion. Without the reduction gears, the high-speed engine or turbine would spin the propeller at too high a rate, resulting in excessive vibration and cavitation, which would be detrimental to the propulsion system. The other answer choices are incorrect because: A) reversing the main engines is a separate system, B) the reduction gears do not directly transmit vibration or thrust to the hull, and D) noise reduction is not the primary purpose of the reduction gears.