Which of the following components is used to prevent sewer gas from entering a building?

The component used to prevent sewer gas from entering a building is the trap. Traps are key fixtures in plumbing systems, designed to hold a small amount of water, creating a barrier that stops sewer gases from escaping into the living spaces of a building. This water seal is crucial for maintaining indoor air quality and preventing unpleasant odors associated with sewer gas.

While the soil pipe is part of the drainage system that carries waste away, and the stack vent helps with air circulation within the plumbing system, neither of these components are specifically intended to block sewer gases as effectively as a trap. The drainage system itself is broader, encompassing various pipes and fixtures, but does not individually serve the same purpose of creating a barrier against sewer gases.