Among the electrolytes of ${\mathrm{Na}}_2 {\mathrm{SO}}_4, {\mathrm{CaCl}}_2,$ ${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3$ and ${\mathrm{NH}}_4\mathrm{Cl}$, the most effective coagulating agent for ${\mathrm{Sb}}_2{\mathrm{S}}_3$ sol is

 Higher is the charge, higher is the coagulating power.

${\mathrm{Sb}}_2{\mathrm{S}}_3$​ sol is negatively charged sol. It will be coagulated by positively charged ions.

In ${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3$, Al³⁺ ion has maximum charge among the given electrolytes. 

Hence, option C is the correct answer.