In what type of check valve does the disk move up and down due to changes in fluid pressure?

The correct choice is the lift check valve, which operates with a disk that moves vertically in response to changes in fluid pressure. In a lift check valve, the fluid flow lifts the disk off its seat when the flow is in the designated direction, allowing for the passage of fluid. When the flow reverses, the disk returns to its seated position, preventing backflow. This mechanism is particularly effective in systems where there is a need to minimize pressure loss and ensure quick valve response to fluid dynamics.

In contrast, ball check valves utilize a sphere that moves to block flow, while swing check valves employ a hinged disk that swings open or closed based on flow direction. Diaphragm check valves use a flexible diaphragm instead of a disk, responding to pressure changes by flexing rather than moving in an up-and-down motion. This fundamental difference in operation is what clearly distinguishes the lift check valve from the others in terms of how it functions under varying fluid pressures.