Which of the following best describes the role of traps in plumbing?

Traps play a crucial role in plumbing systems primarily by preventing unpleasant odors from entering a building. They are designed to hold a small amount of water, creating a seal that blocks the passage of gases from the sewer or drainage system. This water seal is typically a few inches deep and effectively closes off the drain from the air in the room, thereby ensuring that no foul odors can escape into the living space.

The other options do not accurately reflect the function of traps. Enhancing water pressure is typically the role of pumps or pressure regulators, not traps. While traps may interact with certain substances in a drain, they are not designed to filter out chemicals; this function is more relevant to specialized filtration systems. Lastly, traps do not regulate water temperature; temperature control is managed through other plumbing components, such as water heaters or mixing valves. Therefore, the primary function of traps is indeed to eliminate unpleasant odors, making the selected answer correct.