In the industrial production of which of the following, molecular hydrogen is obtained as a
byproduct?

Sodium hydroxide is generally prepared commercially by electrolysis of sodium chloride in castner Kellner cell.

$$\begin{gathered} \text{ at cathode : }\mathrm{Na}+{\mathrm{e}}^{-}\stackrel{\mathrm{Hg}}{⟶}\mathrm{Na}-\text{ amalgum } \\ \text{ Anode }:{\mathrm{Cl}}^{-}⟶\frac{1}{2}{\mathrm{Cl}}_2+{\mathrm{e}}^{-} \end{gathered}$$

The Na–amalgam is treated with water to give sodium hydroxide and hydrogen gas :

$2\mathrm{Na}\left(\text{ amalgam }\right)+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+{\mathrm{H}}_2+2\mathrm{Hg}$