The correct answer is Enamel.

- Enamel is the outermost layer of the tooth and is composed mainly of inorganic hydroxyapatite crystals, making it the hardest dental tissue.
- Importantly, enamel is acellular, meaning it does not contain any living cells.
- Because of this, enamel has no ability to regenerate or repair itself once it is damaged or worn away.
- This distinguishes it from other dental hard tissues such as dentin, cementum, and alveolar bone, which all contain cells (odontoblasts in dentin, cementoblasts in cementum, and osteocytes in bone) capable of limited regeneration or repair.

- Dentin contains odontoblasts that can form secondary dentin.
- Cementum contains cementoblasts and can undergo slow repair and remodeling.
- Alveolar bone is a living tissue with osteocytes and is constantly remodeling.

Reference: Ten Cate’s Oral Histology, 9th Edition, Chapter 3: Tooth Development and Structure / Page 45