Final answer:
The substance that acts as an adhesive in this context is the Dentin bonding agent, which facilitates the bond between the cavity and the composite resin restoration.
Step-by-step explanation:
The substance that serves as an adhesive between the cavity preparation and the composite resin restoration is the Dentin bonding agent. It is applied to the tooth structure to create a micro-mechanical bond between the tooth tissue and the composite resin used in the restoration. Etchant, often composed of phosphoric acid, is used to clean and micro-etch the surface of the tooth, improving the bonding process. The composite resin is then applied over the bonding agent to fill the cavity, and it does not serve as an adhesive itself. Amalgam, on the other hand, is a different type of material commonly used for fillings that includes metals such as silver and mercury, which form a solid solution used to fill dental cavities, but it is not used as an adhesive.