In the commercial electrochemical process for aluminium extraction the electrolyte used is

Question

$ (a)\;\text{$Al(OH)_3$in $NaOH$ solution } \\(b)\;\text{An aqueous solution of $Al_2(SO)_3.$ } \\(c)\;\text{ A molten mixture of $Al_2O_3$ and $Na_3AlF_6$ } \\ (d)\;\text{A molten mixture of $Al_2O_3$ and $Al(OH)_3$ } $