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

The correct choice is that every fixture trap in a plumbing system shall be protected by a vent. Venting is essential in plumbing systems, as it allows air to enter the drainage system, which helps maintain neutral air pressure within the pipes. This balance of pressure is crucial for proper drainage and prevents the vacuum effect that can siphon water out of traps, leading to a loss of trap seal.

The trap seal is the water barrier in the trap designed to prevent sewer gases from entering the living space. Without a proper vent, the trap seal could be compromised, leading to potential health risks from sewer gases and also reducing the overall effectiveness of the plumbing system. By providing a vent, you ensure that the trap maintains its seal while allowing for the necessary air exchange that prevents siphoning.

Having a 3-inch deep trap seal or an anti-siphon device can be beneficial features in specific scenarios, but they do not substitute for the fundamental requirement of proper venting to protect trap seals adequately. Venting is a core aspect of maintaining the functionality and safety of a plumbing system.