Which plumbing term refers specifically to the lowest horizontal portion of a drainage system?

The correct answer is the term that specifically describes the lowest horizontal portion of a drainage system. In plumbing terminology, the "Building Drain" is defined as the part of the drainage system that collects wastewater from various plumbing fixtures and conveys it to the point of disposal, typically leading to the sewer line or septic system. This section is crucial as it is the main horizontal run that gathers effluent from all the vertical stacks and fixtures in a building.

The significance of this term lies in its role in effectively managing and directing wastewater away from the building, thereby preventing potential wastewater backup and ensuring proper sanitation. The Building Drain is foundational to the drainage system, as it connects to lateral lines that ultimately lead to the public sewer or treatment facilities.

In contrast, other terms refer to different components within the drainage system. For example, a Stack Vent refers to the vertical component that allows for the venting of gases and air from the drainage system, while a Fixture Drain pertains to the drainage that connects directly to individual plumbing fixtures. A Waste Stack denotes the vertical pipe that transports waste from toilets or other fixtures, but it does not represent the horizontal section of the drainage system itself. Thus, the specific definition of the Building Drain fits the question perfectly.