$\mathrm{KI}$ solution reacts with excess ${\mathrm{AgNO}}_3$solution. The sol formed is most effectively coagulated by

The precipitate formed is $\mathrm{AgI}$;

$\mathrm{AgI}$is surrounded by excess ${\mathrm{AgNO}}_3$; 

$\mathrm{AgI}$ adsorbs the common ion, ${\mathrm{Ag}}^{+}$ from ${\mathrm{AgNO}}_3$;

A postive sol $\mathrm{AgI}/\mathrm{Ag}+$is formed;

Coagulation power of anion to coagulate a postive sol is

${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-}>{\mathrm{PO}}_4^{3-}>{\mathrm{SO}}_4^{2-}>{\mathrm{X}}^{-}$;

Coagulating value $\propto \frac{1}{\mathrm{Coagulation} \mathrm{power}}$