The seal of every fixture trap in a plumbing system shall be protected by a __________________________.

The protection of the seal of every fixture trap in a plumbing system is crucial for preventing the escape of sewer gases into occupied spaces. A vent plays a central role in maintaining the necessary pressure balance within the plumbing system. By allowing air to enter the plumbing lines, the vent helps prevent the siphoning of water from the trap. This ensures that the trap seal remains intact, as it is designed to block gases while allowing wastewater to flow away.

On the other hand, while some of the other options like trap primers can assist in maintaining the water seal, they do not address the primary function of venting that safeguards against pressure changes that can lead to trap seal loss. Hence, proper venting is essential according to plumbing codes to effectively protect the integrity and functionality of fixture traps.