What type of plumbing fixture is referred to as a frost-proof closet?

The term "frost-proof closet" refers to a specific type of plumbing fixture designed to prevent freezing in cold weather conditions. The correct characterization for a frost-proof closet is one that does not contain water in the bowl and has its trap and water supply control valve located below the frost line. This design is essential because it allows the fixture to function properly even in extreme cold without the risk of the water freezing in the trap or supply lines.

The mechanism works by ensuring that the components which might be at risk of freezing are positioned below the ground level where temperatures are less likely to drop below freezing. By excluding water from the bowl, it further reduces the risk of ice formation, which is critical for maintaining the functionality of the fixture in freezing climates.

The context of other choices provides a better understanding of why this particular answer fits the definition of a frost-proof closet. Some options mention configurations that would not adequately protect the plumbing from freezing temperatures, thus negating their practicality in cold environments.