Explanation

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