Which test requires the drainage system to be filled until water appears at the bottom of the lowest fixture?

The fixture flood test involves filling the drainage system with water until it reaches the bottom of the lowest fixture. This method is specifically designed to check for leaks in the fixtures and connections by ensuring that the water does not drain away, which would indicate a leak. By observing water accumulation at the lowest fixture, this test effectively verifies the integrity of the drainage system and its ability to hold water without any leaks or failures.

In the context of the options, while the fixture leak test primarily assesses individual fixtures for leaks, the drainage pressure test evaluates the system's ability to withstand pressure without leaks. The backflow prevention test focuses on the prevention of contaminated water from flowing back into the system, rather than assessing the drainage capacity itself.