**Core Concept**
The expansion of amalgam alloy is a critical factor in its clinical performance, particularly in relation to the prevention of microleakage and the maintenance of the marginal seal. The expansion of amalgam is influenced by the properties of its constituents, specifically the gamma-2 phase, which is responsible for the alloy's expansion.

**Why the Correct Answer is Right**
The gamma-2 phase is a tin-rich phase that is responsible for the expansion of amalgam alloy. When the gamma-2 phase reacts with mercury, it forms a tin-mercury compound that expands, causing the alloy to expand. However, some constituents of amalgam alloy can decrease this expansion. The correct answer is tin-free silver-tin (Ag-Sn) alloy, which is known to decrease the expansion of amalgam alloy. This is because Ag-Sn alloy has a lower gamma-2 phase content compared to other constituents, resulting in less expansion.

**Why Each Wrong Option is Incorrect**
**Option A:** Silver-tin (Ag-Sn) alloy actually increases expansion due to its high gamma-2 phase content.
**Option B:** Copper (Cu) is not a constituent that affects expansion, but rather is added to amalgam alloy to improve its corrosion resistance.
**Option C:** Tin (Sn) is a key constituent of amalgam alloy, but it is not the one that decreases expansion. In fact, tin is responsible for the formation of the gamma-2 phase, which causes expansion.

**Clinical Pearl / High-Yield Fact**
To remember the constituents of amalgam alloy and their effects on expansion, use the mnemonic "Snakes Go Up": Sn (tin) goes up (causes expansion), while Ag-Sn (silver-tin) goes down (decreases expansion).

**Correct Answer:** C.