A negatively charged suspension of clay in water needs for precipitation the minimum amount of.

The concept used: Surface Chemistry 

Detailed explanation: A negatively charged suspenstion of clay in water needs for precipitation the minimum amount of ${AlCl}_3$ as it has the maximum positive charge as compared to the other salt solutions. Cation with maximum charge will be most effective for coagulation. Coagulating power is directly proportional to the charge of the ion. For negative charged colloid, we see the cation of the salt solution and for the positive charges colloid, we see the anion of the salt solution.

Hence, the correct answer is option A) ${AlCl}_3$.