In Hall - Heroult process, the following is used for reducing $\mathrm{Al}_{2} \mathrm{O}_{3}$ :-

<p>In the Hall-Héroult process, graphite anodes are used to reduce alumina (Al₂O₃). During the electrolytic reduction, Al³⁺ ions are reduced at the cathode to form aluminum, while at the anode, oxide ions are oxidized to form oxygen gas. The oxygen gas then reacts with the carbon in the graphite anodes to produce carbon dioxide. This means that graphite is consumed during the process, and the anodes have to be replaced periodically. </p> <p>So, in the context of the question, graphite (Option C) is the correct answer, as it is the material of the anode used for the electrolytic reduction of Al₂O₃ in the Hall-Héroult process.</p>