Correct Answer: A
Explanation:
In order to make the image permanent, it is necessary to remove the unreduced silver bromide from the surface of the developed film. This operation is called fixing of image. Fixing is done by dipping the developed film or plate in sodium thiosulphate (hypo) solution. the hypo solution dissolves the unreduced silver bromide by forming a complex. Thus sodium thiosulphate acts as a complexing agent.