Arseneous sulphide solution is a negative sol. $ [A{{s}_{2}}{{S}_{3}}]\,\,{{S}^{2-}}. $ Hence, to coagulate it, a cation is needed. According to Hardy-SchuIze law, coagulation power of $ A{{l}^{3+}} $ will be maximum (as it is trivalent). Hence, its amount required will be the least and therefore, its coagulation value will be least.