A _______ prevents sewer gases from emanating from the drainage system.

A trap is a crucial component in plumbing systems that serves the specific purpose of preventing sewer gases from entering a building. It achieves this through a simple design that creates a water seal. When water fills the curved portion of the trap, it blocks gases from flowing back up the drainage system into living spaces, while still allowing wastewater to pass through. This water seal is effective because it utilizes the principle of gravity and fluid dynamics to maintain a barrier against the odor and harmful gases present in sewer systems.

The other options do not fulfill this function effectively. Filters typically serve to remove particles and contaminants from water but do not specifically address gas prevention. Valves are used to control the flow of fluids in a system but do not create a seal needed to prevent gas escape. Joints connect different sections of pipes but do not have the capability of stopping gases from coming back into a space. Thus, the role of a trap in maintaining air quality and safety by preventing sewer gases is paramount.