**Core Concept**
The question is testing the radiopacity of various dental restorative materials. Radiopacity refers to the ability of a material to appear white or light on dental radiographs, making it visible against the darker background of the surrounding tooth structure. This property is crucial for diagnosing and monitoring dental restorations.

**Why the Correct Answer is Right**
The correct answer is silver amalgam. Silver amalgam is a mixture of mercury, silver, tin, and copper that is widely used for filling teeth. Its high mercury content is responsible for its excellent radiopacity, making it easily visible on dental radiographs. The other materials, such as composite resin, glass ionomer cement, and gold, have varying degrees of radiopacity but are generally less radiopaque than silver amalgam.

**Why Each Wrong Option is Incorrect**
**Option A:** Composite resin is a tooth-colored restorative material that is less radiopaque than silver amalgam due to its lack of heavy metal components.
**Option B:** Glass ionomer cement is a type of dental restorative material that is more radiolucent than silver amalgam, making it less visible on dental radiographs.
**Option C:** Gold is a highly radiopaque material, but it is not commonly used for dental restorations due to its high cost and limited durability compared to silver amalgam.

**Clinical Pearl / High-Yield Fact**
When evaluating dental restorations on radiographs, it's essential to remember that silver amalgam is highly radiopaque, while composite resin and glass ionomer cement are less radiopaque.

**Correct Answer:** D. Silver amalgam.