Final answer:
The straight, white border on a photograph is most likely due to uneven development, where some parts of the film do not fully immerse in the developing solution, remaining unexposed and clear post-development.
Step-by-step explanation:
The processing error that causes a straight, white border on a photograph is most likely due to uneven development. During the development process, if parts of the film do not get adequately immersed in the developing solution, those areas will not react chemically and thus remain unexposed, leading to sections of the film that stay clear or white once the process is complete.
Option 1, insufficient fixing time, typically results in a milky or grey discoloration rather than a straight, clear border, and can affect image stability. Option 2, over-fixing, can lead to the degradation of the image but does not cause white borders. Option 4, overexposure during film capture, would result in a photograph that appears overly bright or "blown out" in the overexposed areas but would not directly cause a uniform straight white border. Therefore, uneven development is the most probable cause of a straight, white border on a photograph.