**Core Concept**
Zinc-containing amalgam is a commonly used dental restorative material. The expansion of this material is a critical property that affects its clinical performance and longevity. The expansion is primarily due to the difference in thermal expansion coefficients between the various components of the amalgam.

**Why the Correct Answer is Right**
The expansion of zinc-containing amalgam is caused by the formation of zinc oxide (ZnO) during the setting reaction. When the amalgam is mixed and placed in the mouth, the zinc reacts with the oxygen in the air to form zinc oxide. This reaction releases heat and causes the amalgam to expand. The zinc oxide also reacts with the other components of the amalgam to form a strong and durable restoration.

The correct answer is the one that identifies the formation of zinc oxide as the cause of expansion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not mention the formation of zinc oxide, which is the primary cause of expansion in zinc-containing amalgam.
**Option B:** This option is incorrect because it refers to the expansion of the amalgam due to the difference in thermal expansion coefficients, which is a secondary effect.
**Option C:** This option is incorrect because it mentions the reaction of zinc with copper, which is not relevant to the expansion of zinc-containing amalgam.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the expansion of zinc-containing amalgam is a critical property that affects its clinical performance and longevity. The formation of zinc oxide during the setting reaction is a key factor in this expansion, and dentists should be aware of this property when selecting and placing dental restorations.

**Correct Answer:** A. Zinc oxide formation during the setting reaction.