The chemical used as a ‘fixer’ in photography is

In photographic processing the fixer removes unreacted silver halide crystals by forming soluble silver–thiosulfate complexes that can be washed away. Common thiosulfate salts used as fixers are sodium thiosulfate (historically called “hypo”) and ammonium thiosulfate; sodium thiosulfate is widely cited as the traditional photographic fixer and is effective at dissolving remaining silver halides to prevent further darkening or fogging of the image [1]. Sources describing fixation chemistry and common fixer salts explicitly list sodium thiosulfate as a primary fixer used for film and paper, confirming option 2 as correct [1].

Sources

1. [1] https://en.wikipedia.org/wiki/Photographic_fixer