The correct answer is (a) Thiols
The best I can explain: Thiols are used as preservatives in liquid enzyme preparations to create a reducing environment. Enzymes are very much more stable in the dry state than in solution. Solid enzyme preparations sometimes consist of freeze-dried protein with inert materials such as starch, lactose, carboxymethylcellulose and other poly-electrolytes that protect the enzyme during a cheaper spray-drying stage.