The minimum concentration of an electrolyte in millimoles per litre required to cause coagulation of a sol in two hours is called

"The number of milligrammes of the protective colloid which must be added to 10 cc of a specific gold sol so as to merely avoid its precipitation by addition of 1 cc of 10% $\mathrm{NaCl}$ solution," is how Zsigmondy defined the gold number. 

Only when an electrolyte has a specific minimum concentration does it cause colloids to coagulate. 

The smallest amount of electrolyte, measured in millimoles, that must be supplied to one litre of colloidal sol in order to achieve 

Hence the correct option is (A).