We need the chemical that fixes a photographic image by removing leftover silver halide.
- Sodium sulphate is just a simple salt with no ability to dissolve silver halide, so it cannot fix a film.
- Borax is a mild alkaline cleaning agent, used in glass and detergents, not in photography processing.
- Ammonium sulphate is a common nitrogen fertiliser and has no role in film development.
- Sodium thiosulphate reacts with silver bromide to form a soluble silver thiosulphate complex, $\mathrm{AgBr + 2Na_2S_2O_3 \to Na_3[Ag(S_2O_3)_2] + NaBr}$, which washes off the film and leaves a stable image.
So option A, sodium thiosulphate, is the fixer used in photography.