During the electrolytic reduction of alumina, the reaction at cathode is :

Question

( A ) $2H_2O \to O_2 + 4H^+ + 4e^-$
( B ) $3F^- \to 3F + 3e^-$
( C ) $2H^+ + 2e^- \to H_2$
( D ) $Al^{3+} + 3e^- \to Al$