The correct answer is A. The reduction gear allows the prime mover (such as a diesel engine or gas turbine) 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 propellers operate most efficiently at lower rotational speeds and higher torque, while prime movers operate most efficiently at higher rotational speeds and lower torque. The reduction gear acts as an intermediary, allowing the transfer of power from the high-speed, low-torque prime mover to the low-speed, high-torque propeller. 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.