(D) $NaOH$ is prepared by the electrolysis of brine in the Castner-Kellner cell.In this cell,a mercury cathode is used.The reactions are as follows:At

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(D) $NaOH$ is prepared by the electrolysis of brine in the Castner-Kellner cell.In this cell,a mercury cathode is used.The reactions are as follows:At anode: $2Cl^{-} \to Cl_2 + 2e^{-}$At cathode: $Na^{+} + e^{-} + Hg \to Na-Hg$ (amalgam)The sodium amalgam is then treated with water to produce $NaOH$:$2Na-Hg + 2H_2O \to 2NaOH + 2Hg + H_2 \uparrow$