What is the alum used for the purification of water?

Given below are two statements:
Statement $I:$ In the coagulation of a negative sol,the flocculating power of the three given ions is in the order - $Al^{3+} > Ba^{2+} > Na^{+}$
Statement $II:$ In the coagulation of a positive sol,the flocculating power of the three given salts is in the order - $NaCl > Na_{2}SO_{4} > Na_{3}PO_{4}$
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following electrolytes will have maximum coagulating value for $AgI / Ag^{+}$ sol?

Which of the following is lyophobic in nature?

Identify the method of preparation of a colloidal sol from the following.

The minimum concentration of an electrolyte in $mol \ L^{-1}$ required to cause coagulation or precipitation of a sol is known as: