Which part of the prophet cup is responsible for polishing the tooth?

The edge of the cup is responsible for polishing the tooth because it is specifically designed to come into contact with the surfaces of the tooth during dental procedures. The polished edge effectively removes plaque and tarnish, creating a smooth surface on the tooth enamel. This polishing action is essential in dental care as it helps enhance the aesthetics of the teeth by making them appear cleaner and brighter.

In contrast, the other parts of the cup, such as the center, underside, and outer surface, do not contribute to the polishing action effectively. The center is typically not utilized for the actual polishing process, while the underside and outer surface may serve different functions or be less direct in contact with the tooth. Thus, the edge of the cup is specifically engineered for the task of polishing, making it the correct answer.