**Core Concept**
The Hg alloy ratio in dental amalgam refers to the proportion of mercury (Hg) in the amalgam alloy. The correct composition of dental amalgam is a critical factor in determining its physical and mechanical properties, such as durability and resistance to corrosion.

**Why the Correct Answer is Right**
A higher Hg alloy ratio in dental amalgam increases its fluidity and ease of condensation, allowing for better adaptation to the cavity preparation. This is because mercury acts as a solvent, dissolving the other alloy components and facilitating the formation of a strong and durable restoration. The optimal Hg alloy ratio is typically around 50-55%, which balances the benefits of increased fluidity with the potential risks of mercury toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** A lower Hg alloy ratio may result in a more brittle and prone-to-fracture restoration, as the reduced fluidity makes it more difficult to achieve a good seal with the cavity walls.
**Option B:** A Hg alloy ratio of 100% would be impractical and potentially toxic, as pure mercury is not suitable for use in dental amalgam.
**Option C:** A Hg alloy ratio of 0% would result in a restoration that is too hard and brittle, lacking the necessary fluidity and adaptability to withstand the stresses of chewing.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the use of dental amalgam is being phased out in many countries due to concerns about mercury toxicity and the availability of more modern and mercury-free restorative materials.

**Correct Answer: D.**