Which of the following electrolytes will be most effective ion in the coagulation of gold sol 

Greater is the valency of the oppositely charged ion of the electrolyte is added, the faster is the coagulation. Gold sol is an anionic (negative) sol. So, the coagulating power of an electrolyte for gold sol depends on the coagulating power of cation which is directly proportional to the charge of the cation.

In the given options, Mg has highest cationic charge in MgCl₂, therefore option-d is the correct option.