Which of the following instruments is used to retain a rubber dam in place?

The rubber dam frame is specifically designed to retain a rubber dam in place during dental procedures. This instrument provides support and stability to the rubber dam, ensuring that it remains securely positioned around the tooth or teeth being treated. The frame helps hold the dam taut, which is essential for creating an isolated working field free from saliva and debris.

The other options serve different purposes in dental practice. For example, a rubber dam punch is utilized to create holes in the rubber dam for the teeth being treated, while a matrix band is used in restorative dentistry to provide a temporary wall for filling a tooth. A mixing pad is employed for combining dental materials but does not contribute to the retention of a rubber dam. Thus, the function of the rubber dam frame makes it the correct answer for supporting and retaining the rubber dam during treatment.