Which type of vent connects at the junction of two fixture drains and serves as a vent for both fixtures in a vertical position?

The common vent is the correct choice because it is specifically designed to serve two fixtures that are connected at the same junction within the plumbing system. This type of vent operates in a vertical position, allowing it to effectively air out drainage from both fixtures simultaneously. Its primary function is to equalize pressure within the drainage system and prevent siphoning, which can occur when one fixture is used and the water flow creates a vacuum that disrupts the drainage of the other fixture.

In comparison, an individual vent serves a single fixture and does not connect multiple drains; rather, it ensures that just one fixture retains adequate airflow. A stack vent is used on the vertical section of the drainage stack that extends up through the roof, primarily allowing for venting services at various fixture levels and is not limited to just two connected fixtures. A branch vent, on the other hand, is used to vent horizontal runs of the drainage system but typically does not operate at the junction of two fixture drains.

Thus, the common vent is uniquely suited for its role in connecting and serving two fixtures efficiently, which is why it is the correct answer.