This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Composition of fixer solution: 
Select and explain. 
A. Na thiosulfate, acetic acid,  Na2SO4, KBr, B. Na thiosulfate, acetic acid,  Na2SO4, Al, C. Hydroquinone, acetic acid, Na2SO4, KBr, D. Hydroquinone, acetic acid,  Na2SO4, Al
B: Na thiosulfate, acetic acid,  Na2SO4, Al
Answer: B: Na thiosulfate, acetic acid,  Na2SO4, Al 
Explanation: Fixing solution also contains four components, all dissolved in water:  (1) clearing agent, (2) acidifier,  (3) preservative, and (4) hardener.
 
Clearing Agent
An aqueous solution of ammonium thiosulfate (“hypo”) dissolves the unexposed silver halide grains.
 
Acidifier
The fixing solution contains an acetic acid buffer system (pH 4 to 4.5) to keep the fixer pH constant.
 

Preservative
Ammonium sulfite is the preservative in the fixing solution, as it is in the developer.

 
Hardener
A hardening agent, usually aluminum sulfate, complexes with the gelatin during fixing and prevents damage to the gelatin during subsequent handling. 
 
 
 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 70