- When the dental pulp is exposed to stimuli such as dental caries, the tooth responds by producing a specific type of dentin to protect the pulp tissue.
- This dentin is known as tertiary dentin.

- Primary dentin is the dentin formed during the initial development of the tooth before eruption.
- It provides the basic structure of the tooth.

- Secondary dentin is gradually deposited after tooth eruption throughout life and is responsible for the slow reduction of the pulp chamber size over time.

- In contrast, tertiary dentin, also called reparative or reactive dentin, is formed rapidly in response to external insults such as dental caries, trauma, or restorative procedures.
- Its primary function is to protect the dental pulp by creating a barrier to limit the progression of injury.

Predentin is the unmineralized organic matrix secreted by odontoblasts, which later mineralizes to form mature dentin.

Therefore, the correct answer is tertiary dentin because it is specifically produced as a defensive mechanism against dental caries.

Reference: Ten Cate's Oral Histology, 9th Edition, Chapter 5: The Dental Pulp and Dentin, Page 123-125