How do check valves prevent reverse flow?

Check valves are designed to allow fluid to flow in one direction while preventing it from flowing backward, effectively maintaining the intended flow direction in a pipeline or system. The correct answer highlights that these valves can utilize both a mechanical component, such as a ball or flap, which closes in response to reverse fluid flow and the force of gravity to enhance that closure action.

When reverse fluid flow occurs, the ball or flap within the valve is pushed against its seat, sealing the opening and stopping the backflow. Gravity can assist in this process; particularly in vertically oriented installations, it can help ensure that the ball or flap remains in a closed position when the flow stops or reverses, further enhancing the valve's effectiveness.

Thus, option D accurately encapsulates the dual mechanisms by which check valves function to prevent reverse flow, demonstrating the versatility in design and functionality of these important components in fluid systems.