Which of the following causes films to smell and turn yellow over time?

Weak fixer solution can cause films to smell and turn yellow over time due to its inability to effectively remove residual silver halides from photographic emulsion. When the fixer solution is diluted or has reduced strength, the undeveloped silver halides remain on the film after processing. This residual material can lead to chemical reactions, eventually resulting in discoloration and unpleasant odors. Yellowing occurs as the organic components in the emulsion break down or react with environmental factors, producing noticeable changes in the film's appearance.

Other factors like cold wash water or poor water circulation may contribute to film processing issues, but they do not directly cause the specific smell and yellowing associated with weak fixer solutions.