Which device prevents sewer gases from entering a plumbing system while allowing sewage to flow?

The device that prevents sewer gases from entering a plumbing system while allowing sewage to flow is the trap. Traps are typically U-shaped pipes located underneath sinks, toilets, and other plumbing fixtures. The design of the trap retains a small amount of water, which acts as a barrier against sewer gases that could otherwise escape into living spaces. Additionally, traps permit the passage of sewage and waste materials down the drainage system, ensuring both safety and functionality.

While cleanouts, vent stacks, and drain fields serve important roles in a plumbing system, they do not specifically function to retain water and block sewer gases in the same way that traps do. Cleanouts provide access to the drainage system for maintenance and clearing clogs. Vent stacks allow air to enter the plumbing system, helping to maintain proper pressure and facilitating drainage but do not prevent the entry of gases. Drain fields are part of septic systems designed to treat wastewater but do not directly serve to block gas entry into a home. Thus, traps are specifically designed for the crucial function of preventing sewer gas while allowing waste to flow.