Final answer:
The matrix retainer is the tool used to keep the matrix band in place during a class II restoration. It holds the matrix band firmly to the tooth to help define the shape of the restoration, while the wedge and T-band are related tools with different uses.
Step-by-step explanation:
The tool used to keep the matrix band in place during a class II restoration is the matrix retainer. A matrix band is a thin, flexible band that is placed around a tooth to form a mold around which a dental composite or amalgam can be packed during the restoration of a cavity. The matrix retainer is specifically designed to hold the matrix band snugly against the tooth to restore its natural shape and to prevent the filling material from expanding outside the prepared cavity.
While options such as a wedge, T-band, and composite resin are also related to dentistry and cavity restorations, they serve different purposes in the process. The wedge is used to adapt the matrix band to the contours of the tooth and to separate the teeth slightly during the procedure. The T-band is another type of matrix band that can be used, especially for primary teeth. Composite resin, on the other hand, is one type of restorative material that might be applied within the matrix band.