The correct answer is A. The reduction gear allows the prime mover (such as a diesel engine) to rotate efficiently at relatively high-speed and low torque, while allowing the propeller to rotate efficiently at relatively low-speed and high torque. This is because the propeller requires high torque to generate the necessary thrust, but operates most efficiently at a lower rotational speed. The reduction gear enables the prime mover to operate at its optimal high-speed, low-torque range, and then transfers that power to the propeller at the appropriate low-speed, high-torque configuration. The other options are incorrect because they do not accurately describe the function of the reduction gear in matching the operating characteristics of the prime mover and the propeller.