Concept:Impurities are separated from bauxite before electrolysis using differences in solubility.Explanation:Bauxite contains silica as an impurity.It is treated with hot sodium hydroxide solution.Aluminium oxide dissolves, while silica and iron(III) oxide remain undissolved.These undissolved impurities are removed as a solid residue.The residue is separated from the solution by filtration.Thus, silica is removed by filtration.Answer:B. filtration