Which of the following methods are used to purify the impure samples of alum?

Crystallisation is based on the difference in the solubilities of the components in a common solvent. Using this method, pure solids are obtained in the form of crystals while impurities remain dissolved in the solvent.

Now, pure alum is soluble in water while its impurities are insoluble in water. Thus, crystals of pure alum are obtained from the impure sample by the method of crystallisation.