The correct option is A K4[Fe(CN)6]
According to Hardy-Schulze Rule, the greater the valency of the flocculating ion added, the greater is its power to cause coagulation.
When KI solution is added to excess AgNO3 solution
AgNO3(aq)+KI(aq)→AgI(s)↓+AgNO3(aq)
AgI gets precipitated.
The precipitated silver iodide adsorbs Ag+ions from the dispersion medium and positively charged colloidal sol is formed (AgI/Ag+).
So anion with greater valency will have greater power to cause coagulation.
Among the given electrolytes, K4[Fe(CN)6] have the higher valence anion [Fe(CN)6]4−
∴ Most effective electrolyte in coagulation of above solution is K4[Fe(CN)6].