In Down’s method for the extraction of sodium, the melting point the electrolyte is lowered by adding:
(a)- Potassium chloride
(b)- Calcium chloride
(c)- Calcium chloride and Potassium fluoride
(d)- Potassium fluoride

Hint: Sodium is extracted from sodium chloride. The melting point of sodium chloride is very difficult to attain. To overcome this mixture of two salts were added. One is of potassium and the other is of calcium.

Complete step by step answer:
Sodium is extracted by the electrolysis process of fused sodium chloride by a process called Down’s process.
Difficulties encountered: Although sodium chloride is very cheap and is abundantly available yet the development of Dow’s process for the extraction of sodium from fused sodium chloride was delayed because of the following reasons:
i)- Sodium chloride melts at 1084 K and it is difficult to attain and maintain the high temperature.
ii)- Sodium boils at about 1156 K and hence at the temperature of electrolysis, the metal liberated will vaporize.
iii)- Molten sodium forms a metallic fog (colloidal solution) with fused sodium chloride.
iv)- The products of electrolysis, sodium, and chlorine, corrode the material of the cell at this high temperature.
In 1924 J.C. Down overcame all these difficulties. He observed that the addition of calcium chloride and potassium fluoride lower the melting point of sodium chloride to 850-875 K.
Sodium is now obtained by electrolysis of a fused eutectic mixture of sodium chloride in a Down’s cell at 873 K using graphite anode and iron cathode.
\[NaCl\to N{{a}^{+}}+C{{l}^{-}}\]

Hence, the correct answer is option (c)- Calcium chloride and Potassium fluoride.

Note: In the Down’s process sodium is liberated at the cathode and the chlorine gas is evolved at the anode.
\[\begin{align}
  & At\text{ }cathode:\text{ }N{{a}^{+}}+{{e}^{-}}\to Na \\
 & At\text{ }anode:\text{ }C{{l}^{-}}\to Cl+{{e}^{-}} \\
\end{align}\]
 In the eutectic mixture sodium chloride was 40% and calcium chloride was 60%.