A class II cavity has been prepared for amalgam restoration in an upper molar tooth. Which one of the following items must be used by the operator before the filling can be placed?

In preparing a class II cavity for an amalgam restoration, the use of a matrix outfit is essential. The matrix provides a temporary wall to support the restoration material as it is placed in the cavity. In the case of a class II restoration, which often involves the interproximal surface of the tooth, the matrix allows for the filling material to be shaped correctly and ensures that the restoration has the proper contour and contact with adjacent teeth once completed.

A matrix outfit typically consists of a matrix band and a retainer, which work together to secure the band around the tooth. This setup is critical in establishing a well-formed anatomical shape for the filling, preventing overflow of the amalgam into the surrounding tissues and allowing for optimal function and aesthetics.

While other items like articulating paper, gingival retraction cord, and wooden wedges have their roles in dental procedures, they do not serve the same fundamental purpose of shaping the restoration and providing support during the filling process. After the use of a matrix is established, tools like articulating paper may be used for evaluating the occlusion of the restored tooth, but they do not precede the actual placement of the filling material.