**Core Concept**
The set amalgam is a dental restorative material composed of mercury, silver, tin, and copper. Its strength is crucial for withstanding masticatory forces. The **amalgam** has different phases, each with distinct properties.

**Why the Correct Answer is Right**
The weakest phase of the set amalgam is the **gamma-2 phase**, which is rich in tin and silver. This phase is prone to corrosion and has lower strength compared to other phases. The **gamma-2 phase** is formed due to the reaction between silver-tin particles and mercury.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the weakest phase.
**Option B:** Incorrect as it is not the weakest phase.
**Option C:** Incorrect as the **gamma-2 phase** is the weakest.

**Clinical Pearl / High-Yield Fact**
The **gamma-2 phase** is the weakest link in the set amalgam, and its presence can lead to decreased strength and increased susceptibility to corrosion.

**Correct Answer:** D. gamma-2 phase.