Final answer:
The radiopaque outline of the bone along the root is cementum, which appears radiopaque due to its density and is essential for anchoring the tooth in the jaw via the periodontal ligament.
Step-by-step explanation:
The thin radiopaque outline of the bone along the root in a radiographic image is cementum. This radiopacity is observed because cementum is a bone-like layer covering the dentin in the root of the tooth and appears brighter on radiographs due to its density compared to surrounding tissues. The cementum's primary function is to aid in the attachment of the tooth to the surrounding alveolar bone via the periodontal ligament.