The correct option is C SiO2
Along with Al2O3 ,SiO2 too dissolves forming NaSiO3.
In bauxite impurities of Fe2O3, TiO2 and SiO2 are present, Fe2O3 and TiO2 are basic oxides therefore does not reacts with or dissolve in NaOH whereas SiO2 is acidic oxide it gets dissolve in NaOH, hence leach out
SiO2+2NaOH→Na2SiO3(aq.)+H2O