For extraction of sodium from $\mathrm{NaCl}\text{, }$ the electrolytic mixture  $\mathrm{NaCl}+\mathrm{KCl}+{\mathrm{CaCl}}_2$ is used. During extraction process, only sodium is deposited on cathode but not K and Ca because :

sodium is deposited on the cathode but not K and Ca because the discharge potential of Na ' is less than  that of  ${\mathrm{K}}^{+}$ and ${\mathrm{Ca}}^{2+}$ ions 

K and Ca discharge since they are more reactive than Na and have a lower potential for discharge. 

Hence Correct option is D