Which type of device is intended to perform a special plumbing function and may operate automatically?

The correct answer is a plumbing appliance, which is specifically designed to perform a particular function within a plumbing system, often operating automatically. These devices are integral to managing water supply, waste removal, or other plumbing tasks effectively and efficiently. Examples include dishwashers, washing machines, and sink disposers, all of which perform plumbing functions while incorporating automatic operation features for optimal convenience and effectiveness.

Plumbing tools, while essential for installation, maintenance, or repair tasks, do not perform functions themselves—rather, they assist the plumber in executing a task. A water heater, while it is an appliance, is a specific type that heats water and does not encompass the broader definition intended in the context of special plumbing functions. Mechanical devices in plumbing can refer to various items including valves and pumps but do not inherently imply automated operation as an appliance does. Hence, plumbing appliances stand apart as the most fitting answer to the question.