Final answer:
A cavity liner or varnish used on dentinal tubules to reduce microleakage is a sealer, distinct from bases, bonding agents, and desensitizers.
Step-by-step explanation:
A cavity liner or varnish that is used on the dentinal tubules to reduce microleakage is most accurately described as a sealer. When applied, sealers form a protective layer that can prevent bacteria and fluids from entering the space between the tooth and the restoration, which can help to reduce sensitivity and secondary caries. They are different from bases, which provide thermal insulation or support for a dental restoration; bonding agents, which promote adhesion between the tooth surface and the restoration; and desensitizers, which are primarily used to treat tooth sensitivity directly rather than to seal against microleakage.