The correct answer is Tertiary dentine.

Dentine is a mineralized tissue that forms the bulk of a tooth beneath the enamel and cementum. It is produced by odontoblasts and can be classified into different types based on its time of formation and function:

1. Primary dentine is the dentine formed during the initial development of the tooth before it erupts. It provides the basic structure of the tooth.

2. Secondary dentine is formed after tooth eruption and continues to be deposited slowly throughout life as a normal physiological process to compensate for wear and tear.

3. Tertiary dentine, also called reparative or reactive dentine, is produced as a defensive reaction by odontoblasts or newly differentiated odontoblast-like cells in response to stimuli such as dental caries, trauma, or physical injury. This dentine forms rapidly at the site of injury to protect the pulp by creating a barrier against further insult.

4. Mantle dentine is the first layer of dentine formed immediately adjacent to the enamel or cementum and has distinct structural characteristics.

Therefore, tertiary dentine plays a critical role in the protective mechanism of the tooth by forming in response to injury or caries.

Reference: Ten Cate's Oral Histology, 8th Edition, Chapter 3: Dentin and Pulp