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

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

$\mathrm{AgI}$is surrounded by excess $\mathrm{KI}$; 

$\mathrm{AgI}$ adsorbs the common ion, ${\mathrm{I}}^{-}$ from $\mathrm{KI}$;

A negative sol $\mathrm{AgI}/{\mathrm{I}}^{-}$is formed;

Coagulation power of cation to coagulate a negative sol is

${\mathrm{Al}}^{+3}>{\mathrm{Ba}}^{+2}>{\mathrm{M}}^{+}$;