Final answer:
A Dual Vent or Unit Vent/Common Vent is a single-vent plumbing system allowing multiple fixtures to connect to one vent stack, saving space and materials while preventing sewer gas release.
Step-by-step explanation:
The term Dual Vent or Unit Vent/Common Vent typically refers to a type of ventilation system used in plumbing. It is actually option 2: A ventilation system with a single vent. This system allows fixtures that are close to each other to be connected to the same vent stack. The purpose of a dual vent is to help prevent water seals in the plumbing traps from being siphoned off, which can lead to the release of sewer gases into the building. It's essentially a way to save space and materials while ensuring that the system functions correctly and adheres to building codes.