In the first stage of the Bayer process bauxite is mixed with a strong base. What is the purpose of the strong base?

Strong base favors the formation of a soluble anion of the aluminum, leaving the silicates and other impurities as precipitates to be removed by filtration.

Strong base dissolves the silicate constituents of the ore and precipitates the aluminum as Al(OH)3 for collection by filtration.

The strong base, along with HF, is used to extract the aluminum from the ore as cryolite, a compound used in the electrolytic reduction of aluminum.

Strong base dissolves the silicates and aluminum constituents of the ore, allowing for the removal of any insoluble impurities by filtration and eventual precipitation of the aluminum from the filtrate.

Strong base dissolves the silicate constituents of the ore, leaving pure aluminum oxide as a precipitate to be collected by filtration.