## **Core Concept**
Tertiary dentin is a type of dentin formed in response to stimuli such as caries, wear, or trauma. It can be classified into two subtypes: reactionary dentin and reparative dentin. The key distinction between them lies in their formation process and cellular involvement.

## **Why the Correct Answer is Right**
Reparative dentin, a type of tertiary dentin, is characterized by the presence of cellular inclusions. It is formed by newly differentiated odontoblast-like cells or stem cells from the dental pulp. These cells produce dentin matrix, which may contain cellular inclusions, making the dentin cellular. This process occurs in response to severe stimuli where the original odontoblast layer is destroyed.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Reactionary dentin is formed by the continued activity of existing odontoblasts. It typically does not contain cellular inclusions because it is produced by the existing odontoblast layer without cellular differentiation or inclusion.
- **Option B:** This option might refer to another dental structure or type of dentin but is not specified. Without details, we focus on the correctness of Option D.
- **Option C:** This could potentially refer to primary or secondary dentin, both of which are primarily acellular.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the presence of tertiary dentin, especially the reparative type, indicates a previous pathological process such as deep caries. This is crucial for dental diagnosis and treatment planning.

## **Correct Answer:** D. Reparative dentin.