Which of the following can produce film fog?

Film fog refers to unwanted exposure on radiographic film that causes a loss of contrast and detail, ultimately leading to poor image quality. Each of the listed options can indeed contribute to film fog in different ways.

A wristwatch can emit low levels of radiation or light from its luminous hands or markers, which can fog unexposed film if they are in close proximity for a duration of time.

Exposed LED electronics can be problematic as they may emit visible or near-visible light that can leak into the darkroom, causing film fog when the film is not properly shielded from this light.

Smoking in the darkroom can also create issues, as the smoke particles can settle on the film or in the processing chemicals, which may compromise the film's development and contribute to fogging.

Since all these factors can lead to unwanted exposure and thus film fog, the correct choice embraces the understanding that multiple potential sources of fog exist within a darkroom environment.