**Core Concept**
The expansion of amalgam alloy is a critical factor in its clinical success, as excessive expansion can lead to microleakage, secondary caries, and restoration failure. The constituents of amalgam alloy play a significant role in determining its expansion properties.

**Why the Correct Answer is Right**
The correct answer is Tin (Sn) because it is the constituent that decreases the expansion of amalgam alloy. Tin has a lower atomic volume than the other constituents, which means it occupies less space in the alloy. As a result, the addition of tin to the alloy reduces its overall expansion. This is because the expansion of the alloy is directly related to the atomic volume of its constituents; the larger the atomic volume, the greater the expansion.

**Why Each Wrong Option is Incorrect**
**Option A:** Mercury (Hg) - Mercury is the primary contributor to the expansion of amalgam alloy due to its high atomic volume. It is the correct answer for increasing expansion, not decreasing it.
**Option B:** Silver (Ag) - Silver has a moderate atomic volume, but its addition to the alloy does not significantly affect its expansion properties. It is not the primary contributor to the decrease in expansion.
**Option C:** Copper (Cu) - Copper has a higher atomic volume than tin, which means it would increase the expansion of the alloy, not decrease it.

**Clinical Pearl / High-Yield Fact**
Remember the "Expansion Order" of amalgam alloy constituents: Mercury (Hg) > Silver (Ag) > Copper (Cu) > Tin (Sn). This order reflects the relative atomic volumes of each constituent and their impact on the expansion of the alloy.

**Correct Answer:** C. Tin (Sn)