Which valve would be used to reverse the ship's motion?

The astern throttle valve is used to reverse the ship's motion by allowing steam to flow in a direction that makes the ship back up. When this valve is opened, it redirects the flow of steam to the engine in such a way that the propeller turns in the reverse direction, thereby propelling the ship astern.

This function is critical for maneuvering the ship in tight spaces, allowing for easy docking or avoiding obstacles. The design of the valve and its placement in the ship’s propulsion system are crucial for ensuring the efficient and controlled operation of the machinery during astern motion.

While the ahead throttle valve regulates steam flow to move the ship forward, the main engine control valve manages the overall operation of the engine, and the reversing gear valve works in conjunction with the astern throttle valve but is not solely responsible for reversing motion. Thus, the specific role of the astern throttle valve in directing steam flow for reversing the ship's direction makes it the correct choice.