Which of the following best represents a developer agent used in radiography?

In the context of radiography and film processing, a developer agent is a chemical used to convert the latent image created on the film by exposure to radiation into a visible image through a developing process. Hydroquinone is widely recognized as a primary developing agent in the developer solution.

Hydroquinone functions by reducing the exposed silver halide crystals on the film to elemental silver, which forms the dark areas of the developed image. Its effectiveness in producing high-quality images, along with its ability to work well with other components in the developer solution, makes it a staple in radiographic processing.

Other substances mentioned do not serve the same role in developing films. For example, calcium hydroxide is often used in other applications such as dental materials, but not as a developing agent. Ammonium sulfate is primarily used as an additive in some developers or as an ammonium source but does not develop images on its own. Sodium bicarbonate can function as a buffer in various chemical processes but is not a developer agent either. Therefore, hydroquinone uniquely fulfills the criteria of a developer agent used in radiography.