A common plumbing fixture that relies on siphonage of a trap seal to function properly is a _______.

The correct answer is the water closet because it relies on the siphonage of a trap seal to effectively remove waste from the bowl and ensure proper drainage. When the toilet is flushed, water rushes into the bowl, creating a siphon effect that pulls waste down through the trap. This process also ensures that the trap remains filled with water, which serves as a barrier to prevent sewer gases from entering the living space.

In the case of sinks, bathtubs, and showers, while they do utilize trap seals, their operation and function are somewhat different than that of a water closet. Sinks typically rely more on gravity to drain waste, and while bathtubs and showers also have traps that prevent sewer gases, their primary mechanism does not exclusively rely on siphonage to the same extent necessary for the effective flushing of a water closet. The water closet is specifically engineered to use a siphonic action to facilitate both the removal of solid waste and to maintain the efficiency of the trap seal.