Which agent is primarily used to improve retention in dental bonding?

The agent that is primarily used to improve retention in dental bonding is acid etch. Acid etching involves the application of a phosphoric acid solution to the enamel and dentin surfaces of a tooth. This process creates a roughened surface, which increases the surface area for bonding agents to adhere effectively. The microscopic irregularities produced by the etching process enhance mechanical retention between the tooth structure and the bonding material, which is vital in achieving a durable and long-lasting bond for restorative materials like composites.

While other options have important roles in dental procedures — such as fluoride for remineralization and caries prevention, water for hydration and washing away debris, and silane for coupling agents in bonding to porcelain or glass — they do not serve the primary function of increasing the retention of dental bonding like acid etch does. Thus, acid etch is key in the bonding process due to its effectiveness in preparing the tooth surface for optimal adhesion.