Which component of the developing solution is primarily responsible for contrast in the image?

The component of the developing solution that is primarily responsible for contrast in the image is hydroquinone. This developing agent is known for its ability to produce deeper shades and enhance the differentiation between the various densities in an X-ray image. Hydroquinone reduces exposed silver halide crystals to metallic silver, which directly affects the image density; the more hydroquinone in the solution, the greater the contrast between light and dark areas in the developed film.

In developing solutions, different components serve specific purposes. For instance, sodium sulfite acts primarily as a preservative, helping to maintain the stability of the other developing agents by preventing oxidation. Sodium thiosulfate is a fixing agent used to remove unexposed silver halide from the film after development, ensuring that only the developed image remains visible. Potassium alum could be included in some formulations for various purposes, but it does not play a direct role in enhancing image contrast.

Therefore, hydroquinone is key to achieving the desired contrast in dental radiographs, making it the correct choice.