Which instrument is primarily used to hold fluoride during treatment?

The fluoride tray is specifically designed to hold the fluoride treatment in place during application. It is molded to fit the contours of a patient's arch, ensuring that the fluoride solution remains in contact with the teeth for an effective duration. This optimal contact enhances the fluoride's ability to remineralize enamel and prevent cavities. Using a fluoride tray is essential for maximizing the benefits of fluoride treatments in both pediatric and adult patients.

In contrast, a prophy angle is primarily used for polishing teeth and applying prophylactic paste during dental cleanings; it does not hold fluoride. The XCP ring is a component of a positioning device for radiographs, allowing for accurate and consistent imaging but has no role in fluoride application. A scaler is an instrument used for removing calculus and plaque from teeth and is not intended for use with fluoride treatments. Thus, the fluoride tray is the correct choice for holding fluoride during treatment.