## Core Concept
The question revolves around the use of cavity varnishes and polycarboxylate cement bases in conjunction with dental amalgam restorations, specifically focusing on the sensitivity to cold water and the properties of these materials. The key principle here involves understanding the role of varnishes in protecting the pulp and their interaction with other restorative materials.

## Why the Correct Answer is Right
The correct answer, **D. applied under the polycarboxylate cement base**, is right because when using a polycarboxylate cement base under a dental amalgam restoration, the cavity varnish should be applied under the polycarboxylate cement base. This is because the varnish forms a protective layer on the dentin, which can help in reducing sensitivity. Moreover, polycarboxylate cement adheres well to the tooth structure and has some antibacterial properties, but applying varnish first can enhance the protection of the pulp.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because applying the varnish over the polycarboxylate cement base may not provide the optimal protection to the pulp as the varnish's effectiveness can be compromised by being on top of the cement rather than directly on the dentin.
- **Option B:** This option does not make sense in the context of the question as it does not specify the relationship between the application of varnish and polycarboxylate cement base.
- **Option C:** This option is incorrect because it suggests an alternative that does not directly address the optimal application sequence for using both materials effectively together.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that when using a polycarboxylate cement base with a dental amalgam restoration, especially in cases where there is sensitivity to cold, applying a cavity varnish **under** the polycarboxylate base can enhance pulp protection. This sequence leverages the benefits of both materials: the adhesive and protective qualities of polycarboxylate cement and the protective, possibly desensitizing effects of the varnish.

## Correct Answer: D. applied under the polycarboxylate cement base