Final answer:
The thin, radiolucent line surrounding the cementum of a tooth's root is the Periodontal ligament space, where collagen fibers connect the root to the jaw.
Step-by-step explanation:
The thin, radiolucent line seen surrounding the cementum of the root of the tooth is identified as the Periodontal ligament space. This space reflects the area where the periodontal ligaments, made of collagen fibers, connect the tooth root to the alveolar bone of the jaw. The ligament space appears as a dark line in radiographs because it is less dense compared to other dental structures like the nearby lamina dura, which outlines the socket and provides a sharp contrast, appearing as a dense white line. The correct option from the choices given is A) Periodontal ligament space.