The correct answer is Tertiary dentin.

Dentinal tissue can be classified into different types based on its time of formation and function:

1. Primary dentin is the dentin formed during the initial formation of the tooth before eruption. It constitutes the majority of the dentin in the tooth and provides the main bulk of the tooth structure.
2. Secondary dentin forms after tooth eruption and continues to develop slowly throughout life. It is a physiological process that occurs as a response to normal aging and wear.
3. Tertiary dentin (also called reparative dentin) is produced rapidly in response to localized stimuli such as dental caries, mechanical injury, or restorative procedures. This form of dentin acts as a defensive mechanism to protect the pulp by sealing off areas of injury and reducing further damage or bacterial invasion.
4. Sclerotic dentin is characterized by the mineralization of dentinal tubules usually beneath carious lesions. It is not newly formed dentin but rather aged dentin that has undergone mineral deposition, which increases its hardness and can impede bacterial penetration.

Reference: Ten Cate’s Oral Histology, 9th Edition, Chapter 4: Dentin and Pulp, Page 85-90.