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

Molecular hydrogen is produced as a byproduct in the industrial production of $\mathrm{NaOH}$ by electrolysis of aq $\mathrm{NaCl}$ solution<br/><br/> $\mathrm{NaCl} \rightarrow \mathrm{Na}^{+}+\mathrm{Cl}^{-}$<br/><br/> $$ \begin{aligned} & \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H^{+}}+\mathrm{OH}^{-} \end{aligned} $$<br/><br/> Cathode : $$2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e} \rightarrow \mathrm{H}_{2}+2 \mathrm{OH}^{-}$$ <br/><br/> Anode : $2 \mathrm{Cl}^{-} \rightarrow \mathrm{Cl}_{2}+2 \mathrm{e}$ <br/><br/> $\mathrm{NaOH}$ is crystallised from the remaining part of electrolyte.