The correct answer is At the root apex.

- Cementum is a specialized calcified tissue covering the tooth root, playing a crucial role in anchoring the periodontal ligament fibers to the root surface.
- There are two main types of cementum: acellular cementum and cellular cementum.

- Acellular cementum primarily covers the cervical two-thirds of the root and is involved in tooth attachment. It is formed slowly and contains no cells.
- Cellular cementum is predominantly found at the root apex and in areas of the root furcation. It contains cementocytes within lacunae and is involved in repair and adaptive remodeling. This region is subject to more functional stress and requires the ability to repair and adapt, which cellular cementum facilitates.

Therefore, the root apex is the region where cellular cementum is predominantly located.

Reference: Ten Cate's Oral Histology, 9th Edition, Chapter 7 - Cementum and Periodontal Ligament, Page 155-160