Impurities depressing the freezing point of a solvent is called colligative property . It depends upon no of particles or ions of impurities. When the given compounds are dissolved in water , they break into ions and depress the freezing point . No of ions formed by one molecule of compound AlCl₃ on ionization is highest ( 4 ) . So this compound will have highest strength to lower the freezing point of solvent.
