Which of the following refers to the system that disposes of waste from plumbing fixtures?

The term "drainage system" specifically refers to the network of pipes, fittings, and equipment that carries waste and wastewater from plumbing fixtures to a point of disposal, such as a municipal sewer or a septic system. This system is crucial in maintaining sanitation and hygiene in both residential and commercial buildings. It encompasses all elements involved in the transport of waste away from the source, ensuring that it does not create health hazards or environmental concerns.

In contrast, options like the "drainage fixture unit" relate to a measure used to calculate load requirements on the drainage system rather than referring to the system itself. The "combining system" is a type of drainage system that includes both sanitary and stormwater waste, but it is not the general term for all plumbing waste disposal. The "vent system" serves to provide air circulation within the drainage system to prevent siphoning and maintain proper flow but does not directly deal with the disposal of waste. Thus, understanding that the drainage system is an all-encompassing term that describes the functioning infrastructure for waste disposal is key to recognizing why it is the correct answer.