What device is responsible for propelling a boat forward or backward?

The device responsible for propelling a boat forward or backward is the propeller. A propeller works by converting rotational motion from an engine into thrust, which pushes water behind the boat and propels it in the opposite direction. As the blades of the propeller spin, they create a difference in water pressure, facilitating movement. This is a fundamental component of many types of boats, whether they are powered by gasoline, diesel, or electric engines.

While other devices listed in the choices have important functions in boat operation, they do not directly provide the means for propulsion. The rudder, for example, is essential for steering and directional control, while the tiller serves as a manual control used to steer smaller boats. The throttle controls the amount of power supplied to the engine but does not itself create thrust. In summary, the propeller is the essential device for moving the boat, making it the correct answer in this context.