Every plumbing fixture that is directly connected to the drainage system MUST have

While venting is important for maintaining proper airflow and preventing pressure buildup in drainage systems, not every plumbing fixture requires an individual vent. Many fixtures can share a common vent, as long as the venting system effectively allows for air circulation and proper drainage, thus making this option incorrect.

An air gap is typically used in specific situations to prevent backflow of contaminated water into the potable water supply, such as in dishwashers or sinks. However, it is not a requirement for every plumbing fixture connected to the drainage system, making this choice unsuitable for the question posed.

An overflow is a safety feature that prevents water from spilling over the edges of a fixture, such as a sink or bathtub, but it is not mandatory for all plumbing fixtures. Many fixtures, like toilets, do not require an overflow, thus rendering this choice incorrect.