Final answer:
A wet vent is a plumbing component that serves both as a vent and a drain for various fixtures. The correct option that defines a wet vent is 'c. A vent that also serves as a drain'. This type of vent is essential for efficient plumbing systems alongside ensuring health and safety by maintaining trap seals and equalizing pressure.
Step-by-step explanation:
A wet vent is a plumbing term referring to a pipe that both helps to ventilate the plumbing system and conveys waste water away from fixtures. Contrary to the other options, it is not necessarily about roof protection or remaining perpetually wet, and it isn't defined by being larger than a dry vent. In residential plumbing, a wet vent may serve one or multiple fixtures, and its design must adhere to specific building codes to ensure proper function and prevent dangerous sewer gases from entering living spaces.
The principle behind wet venting is to allow fixtures to share a common vent, which simultaneously acts as a drain for one or more of those fixtures. This method is practical as it reduces the amount of piping needed and can simplify the layout of plumbing in a building.
To summarize and mention the correct option in the final answer, the answer is c. A vent that also serves as a drain. This system has to be properly engineered to maintain the trap seal (preventing sewer gases from entering the home), provide adequate drain flow, and ensure that the venting still performs its primary function of equalizing pressure within the plumbing system.