The seal of every fitting trap in a plumbing system shall be protected by a ________.

The correct answer is the vent, which is essential for maintaining the proper function of plumbing traps. Vents play a critical role in allowing air to enter the plumbing system, which helps equalize pressure. This prevents the trap seal from being compromised due to siphoning or back-pressure, allowing wastewater to flow freely while keeping sewer gases from entering the building.

In the context of plumbing systems, a properly vented trap will maintain a sufficient seal against odors and gases. Without a vent, changes in pressure within the system, such as when nearby fixtures are used or when water drains, can create a siphoning effect that pulls water out of the trap, thus breaking the seal and allowing harmful gases to escape into the living space.

The other options present alternatives that do not serve the same purpose as a vent in protecting the seal of a trap. For example, a drainage fitting is used to connect different parts of the drainage system, but it does not provide the necessary air flow to maintain the trap seal. An anti-siphon device, while helpful in preventing siphoning, typically works independently and may not ensure the same effectiveness in all scenarios as a dedicated vent would. Furthermore, specifying a 3-inch deep seal does not address the issue of maintaining pressure balance