ANS:B - NaOH

In the Bayer's process, bauxite is digested under pressure using NaOH (sodium hydroxide). The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminum oxide), which is the precursor to aluminum metal. In this process, bauxite ore is dissolved in a hot, concentrated solution of sodium hydroxide under pressure. This digestion step dissolves the aluminum-bearing minerals in the bauxite, leaving behind impurities such as iron oxides and silica. The resulting solution, called sodium aluminate, is then clarified and cooled to precipitate aluminum hydroxide, which is further processed to extract pure alumina.