Which of the following would cause a film to be too light?

A film appears too light primarily when it has been underdeveloped. Underdevelopment means that the film has not been exposed to the developer solution for a sufficient amount of time or at the correct temperature. This can result in insufficient chemical reactions taking place, leading to a film that lacks the necessary density and contrast. Essentially, the silver halide crystals in the film do not fully develop into black metallic silver, leaving the film with a lighter appearance.

The other options involve various processing issues, but they either contribute to overexposure or different types of development problems. For instance, overdevelopment or excessive development time would typically cause the film to be too dark, not too light, while inadequate light exposure would result in insufficient exposure prior to development, generally leading to a film that is also too light but not directly connected to the development process itself in the same way.