Magnesium is extracted by electrolyting fused magnesium chloride containing NaCl and 

${\mathrm{CaCl}}_2$ using.

By electrolyzing fused magnesium chloride including NaCI and ${\mathrm{CaCl}}_2$ with an iron container acting as the cathode and a graphite rod acting as the anode, magnesium may be recovered.

Graphite serves as the anode and an iron container serves as the cathode in the electrolysis reduction of ${\mathrm{MgCl}}_2$.Hence Correct option is C