What is that vent pipe system on plumbing which liquid waste flows through?

The correct choice is the wet vent, which plays a critical role in plumbing systems designed to handle liquid waste. A wet vent is a system that allows both waste and venting to occur in the same pipe. This design is particularly beneficial because it simplifies plumbing layouts by reducing the number of pipes needed in the system.

In a wet vent system, the pipe carries waste from fixtures like sinks or toilets, but it also serves as a vent for air to help maintain proper pressure within the plumbing system. This dual function can lead to a more efficient and cost-effective plumbing design, which is vital for ensuring proper drainage and preventing issues like siphoning or drain clogs.

Other terms related to venting, such as stack vent or back vent, refer to different plumbing configurations that do not combine waste flow and venting in the same way as the wet vent does. The stack vent is designed solely to vent the sewage stack, whereas the back vent typically serves to provide relief to fixtures that are not properly vented through the main stack. The yoke vent helps to connect the stacks but does not carry liquid waste. Understanding the purpose and advantage of a wet vent system is crucial for efficient plumbing design and installation.