What type of pipe makes an angle of less than 45 degrees to the horizontal?

The correct answer to the question regarding the type of pipe that makes an angle of less than 45 degrees to the horizontal is "sloped pipe." A sloped pipe refers to any pipe installation that is inclined to facilitate the flow of fluids—especially the drainage of wastewater—using gravity.

Typically, a sloped pipe will have an angle less than or equal to 45 degrees to ensure efficient drainage while still fitting within the parameters of plumbing codes that dictate slope requirements. For example, drainage pipes often have a specified minimum slope to prevent issues such as clogging or pooling of water.

In contrast, a vertical pipe would make a 90-degree angle with the horizontal, while a horizontal pipe is perfectly parallel with the ground at a 0-degree angle. Transverse pipe generally refers to pipes that run perpendicular but can encompass various angles; however, they may still exceed the specified angle in the question. Thus, sloped pipes are specifically designed to meet the criteria of being less than 45 degrees to promote proper function in plumbing systems.