**Core Concept**
The remaining dentin thickness is a critical factor in determining the success of cavity restoration, particularly in dentin protection. It refers to the amount of dentin remaining between the cavity preparation and the pulp, which serves as a protective barrier against bacterial invasion and pulp irritation.

**Why the Correct Answer is Right**
The remaining dentin thickness is crucial in preventing pulp exposure and irritation. When the dentin thickness is reduced, the pulp becomes more susceptible to bacterial toxins and irritants, leading to pulpitis and potentially, pulp necrosis. The remaining dentin thickness also affects the restoration's longevity and success. A minimum thickness of 2 mm is generally recommended to ensure adequate dentin protection and prevent restoration failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Dentin thickness is not directly related to the type of restoration material used (e.g., amalgam, composite, or ceramic). The remaining dentin thickness is a critical factor regardless of the restoration material.

**Option B:** Although a remaining dentin thickness of 1.5 mm may be sufficient for some restorations, it is not the most important threshold for dentin protection. A thinner dentin layer increases the risk of pulp irritation and restoration failure.

**Option C:** While a remaining dentin thickness of 3 mm is generally considered safe, it is not the most critical threshold for dentin protection. A dentin thickness of 2 mm is a more commonly cited threshold for ensuring adequate dentin protection.

**Clinical Pearl / High-Yield Fact**
When determining the remaining dentin thickness, it is essential to consider the patient's overall dental anatomy and the specific tooth being restored. A minimum dentin thickness of 2 mm should be maintained to ensure the success of the cavity restoration and prevent pulp irritation.

**Correct Answer: C. 2 mm**