What is defined as a vertical length of soil or waste stack at least 8 ft in height?

The correct answer to the question is the branch vent. A branch vent is specifically defined as a vertical length of vent pipe that extends from a horizontal drain to the open air or to a vent stack. In plumbing terminology, a branch vent can be considered an extension of a waste stack when it reaches a certain height, which in this case is specified as at least 8 feet.

Understanding the function of a branch vent is crucial, as it serves to allow air to enter the plumbing system and prevent the development of negative pressure. This is essential for maintaining proper drainage and preventing siphoning of water in traps.

Other options may address aspects of plumbing, but they do not specifically refer to the vertical height or function described in the question. A building drain pertains to the piping that conveys wastewater from the building to a sewer, while a building sewer connects the building drain to the municipal sewer system. A cleanout refers to an access point in the plumbing system that allows for cleaning and maintenance, not a measurement of vertical height.