Final answer:
The most likely cause of black spots on a photographic plate is insufficient fixing during the development process, where unexposed silver halides are not thoroughly removed, leading to spots or discoloration.
Step-by-step explanation:
The processing error that causes black spots on a photographic plate or film is often associated with issues in the development process. When considering the options provided:
Over-fixing during processing (Option 1) is not typically associated with black spots, but rather can lead to an overall density change in the image.
Insufficient fixing during processing (Option 2) could lead to black spots, as the unexposed silver halides are not thoroughly removed, potentially resulting in spots or discolouration.
Over-exposure during film capture (Option 3) does not lead to black spots, instead it causes the image to be overly bright or 'blown out,' especially in the highlights.
Under-exposure during film capture (Option 4) results in an image that is overall too dark, or even completely black, but not specifically in spots.
Therefore, the most likely cause of black spots on a photographic plate due to processing error would be insufficient fixing during the development process.