What processing fault is indicated if a periapical view shows a fading image on the light box?

A fading image on a light box when viewing a periapical radiograph typically points to issues related to the processing of the film. In the context of this question, inadequate fixing time is significant because it directly affects how well the image is preserved on the film.

When the fixation process is not carried out for the appropriate duration, the unexposed silver halide crystals that were not developed may not be adequately removed from the film. This can lead to a radiographic image that appears to fade or lose contrast when it is subsequently viewed, as the remnants of unprocessed crystals can scatter light and diminish the clarity of the image.

The other options, while related to film processing, do not specifically cause images to fade in the same manner. For instance, a developer solution that is too cold can lead to underdevelopment of the film, causing an overall dark or insufficiently detailed image rather than a fading appearance. A concentrated developer solution can lead to excessively dark images or artifacts. Under-development may result in an under-exposed appearance, but not necessarily a fading effect. Therefore, inadequate fixing time clearly aligns with the symptom of a fading image.