Which one of the following electrolytes is used in Down's process of extracting sodium metal?

Sodium metal is manufactured by the electrolysis of fused sodium chloride mixed with $\mathrm{KCl}$ and $\mathrm{KF}$. On electrolysis At iron cathode. $\mathrm{Na}^{+}+e^{-} \longrightarrow \mathrm{Na}(s)$ (metallic sodium) At graphite anode $2 \mathrm{Cl}^{-} \longrightarrow \mathrm{Cl}_2(g)+2 e^{-}$ $\mathrm{NaCl}$ melts at $800^{\circ} \mathrm{C}$. It is difficult to attain and maintain its melting point. So, $\mathrm{KCl}$ and $\mathrm{KF}$ are mixed to lower the melting point of $\mathrm{NaCl}$ to about $600^{\circ} \mathrm{C}$. $\mathrm{KCl}$ and $\mathrm{KF}$ are themselves not electrolysed under the voltage conditions used for sodium.