Which of the following best describes the fixture drain?

A fixture drain is best described as a horizontal pipe. In plumbing systems, fixture drains are typically designed to transport wastewater from various fixtures (like sinks, toilets, and bathtubs) away from the fixtures themselves and into the main drainage system. The horizontal design facilitates efficient gravity flow of wastewater, which is essential for proper drainage.

While there can be vertical portions of the drainage system, the term "fixture drain" specifically refers to the part of the system that runs horizontally from the fixture to facilitate drainage. This characteristic is essential in maintaining the effective function of the plumbing system, as proper slope and orientation of horizontal pipes ensure that waste can flow smoothly and prevent blockages.

Other options describe aspects of plumbing systems, but they do not accurately capture the defining feature of a fixture drain. For example, a shallow pipe might refer to a specific design but does not encompass the overall function and structure of fixture drains as effectively as the horizontal aspect does.