What is the main function of a trap in a plumbing system?

The primary function of a trap in a plumbing system is to prevent sewer gases from entering the building. Traps are designed to retain a small amount of water, which creates a seal that blocks gases from the sewer line. This is crucial for maintaining indoor air quality and preventing harmful odors and toxic fumes from infiltrating living spaces.

In addition to their gas-blocking function, traps also help to catch debris and prevent clogs from reaching larger drainage systems. However, the essential characteristic of a trap is its ability to maintain this water barrier.

While traps might have some influence on drainage and can play a minor role in regulating the flow of water by maintaining a certain level, their most significant and defined purpose is to serve as a barrier against sewer gases, which is reflected in the correct answer to the question.