All of the following tissues are radiopaque EXCEPT:

Radiopacity refers to the property of a substance to resist the passage of X-rays, appearing white or light on a radiographic image. Tissues that are dense, such as bone or enamel, typically exhibit radiopaque characteristics.

Enamel is the hardest tissue in the human body and is highly mineralized, making it radiopaque. The cortical plate, which is part of the outer layer of the jawbone, is also dense and therefore appears radiopaque on radiographs. Alveolar bone, which supports the teeth, possesses a significant mineral content, contributing to its radiopacity as well.

In contrast, the pulp chamber is filled primarily with soft tissue, including blood vessels and nerve fibers. This soft tissue does not absorb X-rays to the same extent as the denser mineralized tissues like bone or enamel, and as a result, it appears radiolucent on X-ray images. Radiolucent areas appear darker, indicating less density and greater penetration by X-rays. This distinction is why the pulp chamber is not considered radiopaque, making it the exception among the options provided.