The correct answer is C) low-speed pinion. The low-speed pinion, labeled as "III" in the illustration, is the component that transmits the rotational force from the engine to the reduction gears. This allows the high-speed engine to drive the slower-rotating propeller shaft. The low-speed pinion is a critical component in the propulsion system, as it enables the engine to efficiently transfer power to the propeller. The other options are incorrect because the first reduction gear (A) and second reduction gear (B) are separate components downstream of the low-speed pinion, while the high-speed pinion (D) is the component that meshes with the low-speed pinion to provide the initial reduction in rotational speed.