What is used for venting only and carries no waste?

The option indicating vent stack is correct because a vent stack is specifically designed for venting purposes in plumbing systems. It provides a pathway for air to enter the plumbing system, which helps maintain neutral air pressure within waste pipes, allowing for proper drainage and preventing sewer gases from entering living spaces.

Unlike waste stacks and drainage pipes, which are used to transport waste materials and sewage, the vent stack does not carry any waste; its sole function is to ensure that the plumbing system operates efficiently by facilitating the flow of air. This airflow is essential for the system's overall performance, helping to prevent clogs and maintaining proper drainage function.

Other options represent components involved in the movement of wastewater. A waste stack carries waste materials, while drainage pipes transport fluids, and a seepage pit is used for the drainage of water from septic systems. None of these fulfill the specific function of only venting without carrying waste, thereby reinforcing why the vent stack is the correct answer.