What phenomenon occurs due to vertical downward gravity flow in a stack impacting a horizontal drain?

The correct choice is hydraulic jump, which refers to an abrupt change in the flow regime of a fluid, typically occurring as water flows from a high-velocity, low-pressure state to a lower-velocity, high-pressure state. In the context of plumbing, when water in a vertical stack flows downward due to gravity and impacts a horizontal drain, it can create a sudden change in flow characteristics.

This sudden impact generates turbulence and can result in a significant change in fluid velocity, leading to localized pressure variations. The phenomenon can cause the water to leap up briefly before settling into a low-velocity flow pattern, visually demonstrating the transition that defines a hydraulic jump.

While the other terms, such as hydraulic lift, hydraulic pressure, and hydraulic force, describe related fluid mechanics concepts, they do not accurately capture the specific phenomenon described in this scenario. Hydraulic lift typically refers to the upward movement of fluids, hydraulic pressure involves the force exerted by a fluid at rest or in motion, and hydraulic force concerns the overall forces in any flowing fluid system, but none of these encapsulate the specific dynamics of a jump due to the impact in a gravitational flow setting.