## **Core Concept**
The question tests knowledge of dental materials, specifically bases used under restorations like silver amalgam fillings, and their radiopacity. Dental bases can be radiolucent or radiopaque, which helps in identifying them on dental radiographs.

## **Why the Correct Answer is Right**
The correct answer, **D. Calcium hydroxide**, is a reasonable choice because some calcium hydroxide products are indeed radiopaque due to the presence of radiopaque fillers or the calcium and hydroxide ions themselves. However, the key detail here is to understand which bases are commonly used under silver amalgam restorations and their radiographic appearance. Calcium hydroxide is a commonly used base for its therapeutic and protective properties.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Zinc phosphate cement is not typically used as a base under restorations for therapeutic reasons but rather for luting. It is radiopaque, but not the best choice for a base.
- **Option B:** Glass ionomer cement (GIC) can be used as a base and is known for being radiopaque. However, it's not the only option and might not be the most probable in this context given the specifics of the question.
- **Option C:** Zinc oxide-eugenol (ZOE) is a common base used under restorations, especially in cases where a sedative effect is desired. However, it is generally radiolucent.

## **Clinical Pearl / High-Yield Fact**
A useful point to remember is that **calcium hydroxide** and **glass ionomer cements** are commonly used bases that can exhibit radiopacity. Zinc oxide-eugenol (ZOE) cements are typically **radiolucent**. This knowledge helps in identifying the type of base used under a restoration on a dental radiograph.

## **Correct Answer:** D. Calcium hydroxide.