If a properly exposed film appears completely black, what does this indicate?

If a properly exposed film appears completely black, it indicates that the film has been exposed to light before processing. This phenomenon occurs when the film is inadvertently exposed to ambient lighting conditions, causing every area of the film to be developed fully black upon processing. In the darkroom context, if the film is not handled in total darkness or if there's a light leak in the processing container, this could lead to total exposure.

The other situations described in the other options relate to different problems that would not result in the film being completely black. Underexposure typically results in a film that appears clear or very light because it has not been exposed to enough radiation to generate a sufficient image. Developing for too long may lead to a film that is overly dark but not uniformly black; specific details would remain visible depending on the initial exposure. Conversely, not developing for a sufficient amount of time would result in a film that appears underdeveloped or hazy, rather than completely black. Thus, the scenario of the film being completely black specifically points to light exposure prior to development.