Which of these appears radiolucent?

In dental radiography, radiolucent areas appear dark or black on the radiographs, indicating that those areas have less density and allow more x-ray penetration. Caries, or dental cavities, are characterized by the decalcification of the tooth structure, which means they have a lower density compared to the surrounding healthy enamel or dentin. This loss of mineral content allows x-rays to pass through more easily, resulting in a radiolucent appearance on the radiograph.

In contrast, calculus represents hardened plaque that is denser than tooth structure, appearing radiopaque (light or white) on an x-ray due to its mineral content. A torus, which is a bony growth, is also generally denser than the surrounding tissue and does not permit x-rays to penetrate it easily, making it appear radiopaque. Root tips, being part of the denser tooth structure, are likewise radiopaque. Thus, caries stands out as the only option that demonstrates a radiolucent characteristic on dental radiographs.