Which of the following describes a horizontal pipe?

A horizontal pipe is defined as one that runs parallel to the ground, indicating that it has minimal slope. The correct choice describes a pipe making an angle of less than 45 degrees with the horizontal, which aligns with the definition of horizontal routing in plumbing.

In plumbing practices, while "horizontal" typically refers to a pipe that is level, any angle significantly less than 45 degrees toward a horizontal position indicates that the pipe is not rising or falling drastically, thus retaining most of its horizontal characteristics. This allows for effective drainage and prevents issues such as gravity not being able to draw fluids through the pipe properly.

In contrast, a pipe that is completely vertical (not a horizontal pipe at all) or one that makes an angle greater than 45 degrees would be considered steeply inclined or vertical, while a pipe that is perfectly aligned with the ground would more accurately reflect a zero-degree angle rather than merely less than 45 degrees.