What is placed over calcium hydroxide to mitigate hypersensitivity before applying a composite resin?

The correct choice is primer because primers are specifically designed to enhance the adhesion of composite resins to the underlying tooth structure. When placed over calcium hydroxide, which is often used as a liner to protect the pulp and manage sensitivity, a primer serves to create a compatible interface for the composite resin. This ensures that the materials bond effectively, which is essential for long-lasting restorative work and helps to alleviate any potential hypersensitivity by sealing the dentin tubules.

While varnishes can also be used in some scenarios to cover calcium hydroxide, they are more commonly associated with sealing dentin rather than providing an ideal surface for bonding with composite materials. Composite layers are typically not used as an intermediary, but rather as the final restorative material. Bonding agents, while important in the process, work best when applied over a primer rather than directly over calcium hydroxide. Hence, the logical and scientifically supported choice is primer due to its role in promoting effective bonding and reducing hypersensitivity.