Which situation would lead to a completely clear radiographic film?

A completely clear radiographic film results from the film being processed in a manner that prevents any image formation. This occurs specifically when the film is placed in fixer before it has been developed. The fixer serves to remove unexposed silver halide crystals, which can then lead to a clear film, as the development process has not occurred to form the image.

In the context of the other situations: exposing the film to light due to a darkroom leak or incomplete cassette closure would typically result in a completely black film rather than a clear one, as light exposure generates a dense, dark image. Leaving the collimator open affects the exposure area and may change the image quality, but it does not lead to complete clarity on the film. Each of these scenarios involves different interactions with light and chemicals that lead to various types of exposures and processing outcomes, but only the scenario involving the premature introduction to fixer directly correlates with a completely clear result.