What device prevents waste from slowing down at the base of a stack and forming a slug of water?

The device that prevents waste from slowing down at the base of a stack and forming a slug of water is known as a deaerator. A deaerator functions by removing excess air or gas from waste water systems, which helps to maintain proper flow rates and prevent blockages caused by the accumulation of gases. When waste flows freely without the interference of trapped air or gas pockets, it prevents the formation of water slugs that can disrupt drainage and create potential backups.

In contrast, the other options serve different purposes within plumbing systems. An air admittance valve helps to prevent siphoning in a drainage system by allowing air to enter, thus keeping the system balanced. A drain trap is designed to create a water seal that prevents sewer gases from entering a building, while a backwater valve helps prevent backflow from the sewer into the drainage system. Each of these devices addresses specific issues within plumbing systems but does not primarily focus on preventing the formation of water slugs at the base of a stack.