Which structure allows for the control of a ship's direction while in motion?

The rudder is the structure responsible for steering a ship and controlling its direction while in motion. When a ship moves through the water, the rudder is positioned in the water flow, which creates a difference in pressure on either side of it. This pressure differential results in a force that turns the ship in the desired direction. The larger the rudder and the more effective its design, the more responsive the ship will be to steering commands.

In contrast, while the propeller provides thrust to move the ship forward, it does not control the direction of movement. The waterline refers to the level at which the ship sits in the water and does not have any active role in navigation. Lifelines are safety features used on deck to prevent personnel from falling overboard and have no impact on the ship's steering mechanisms. Thus, the rudder is essential for effective navigation and maneuverability of a vessel.