**Core Concept**
The composite bandage, also known as a composite restoration, is a dental material used to restore and protect teeth. The remaining dentin thickness is a critical factor in determining the need for a composite bandage, as it affects the tooth's structural integrity and susceptibility to decay.

**Why the Correct Answer is Right**
When the remaining dentin thickness is less than 1.5 mm, the tooth is at a higher risk of fracture or decay. In such cases, a composite bandage is required to provide additional support and protection to the tooth. The composite material helps to distribute the masticatory forces, thereby reducing the risk of tooth fracture. The dentin thickness of 1.5 mm is a common threshold for composite restoration, as it balances the need for adequate strength and durability with the risk of tooth fracture.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no specific guideline or recommendation for a composite bandage when the remaining dentin thickness is 2 mm. In fact, a dentin thickness of 2 mm is generally considered sufficient for most dental restorations.
* **Option B:** The remaining dentin thickness of 0.5 mm is extremely thin and would likely require more extensive treatment, such as a full-coverage crown or root canal therapy, rather than a composite bandage.
* **Option D:** There is no specific guideline or recommendation for a composite bandage when the remaining dentin thickness is 3 mm. In fact, a dentin thickness of 3 mm is generally considered sufficient for most dental restorations.

**Clinical Pearl / High-Yield Fact**
The remaining dentin thickness is a critical factor in determining the need for a composite bandage. A general rule of thumb is to use a composite bandage when the remaining dentin thickness is less than 1.5 mm, as this reduces the risk of tooth fracture and decay.

**Correct Answer: C. 1.5 mm**