The ratio is calculated as : $\frac{\text { Reducing Hydrogen }}{\text { OH group }}$
Orthophosphoric acid $=\frac{0}{3}$
Hypophosphorus acid $=\frac{2}{1}$
Phosphorus acid $=\frac{1}{2}$
Pyrophosphoric acid $=\frac{0}{4}$
$(I) \,S _{2} O _{4}^{2-}$
$(I I)\, S _{2} O _{5}^{2-}$
$(III) \,S _{2} O _{6}^{2-}$
$(A)$ $BF _{3}$
$(B)$ $SiCl _{4}$
$(C)$ $PCl _{5}$
$(D)$ $SF _{6}$