Hardy - Schulz rules are based on ____ of electrolyte ions coagulating the colloid.

•  Hardy - Schulz rules are based on charge of electrolyte ions coagulating the colloid.
•  It that the higher is the charge on cation, the higher will be its efficiency to coagulate the colloidal solution. 
• Coagulating power is directly proportional to the charge of the ion. For negative charged colloid, 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 B) Charge.