In photography, a `fixer` is a chemical solution used to make the developed negative image permanent and light-resistant. Let`s look at each of the options mentioned:

Option 1, sodium sulphate, is not used as a fixer in photography. It is a chemical compound which is mainly used in the manufacture of detergents and in the Kraft process of paper pulping.

Option 2, sodium thiosulphate, is the correct answer. It is traditionally used in photographic processing, acting as a fixer to dissolve unexposed silver halides from photographic emulsions, thereby finalizing the image.

Option 3, ammonium persulphate, is not applicable here. It`s a strong oxidizing agent mainly utilized in etching of printed circuit boards and as a polymerization initiator.

Option 4, borax, is a boron compound and a salt of boric acid. It is used in various applications but not as a fixer in photography. It`s often used as a component in glass, ceramics, and detergents.

In conclusion, sodium thiosulphate (option 2) is the chemical used as a `fixer` in the photography domain as it finalizes the image after the development process