Correct choice is (b) Sizing
For explanation I would say: Sizing is the process of adding materials to the paper in order to render the sheet more resistant to penetration by liquids, particularly water. Rosin is the most widely used sizing agent, but starches, glues, caseins, synthetic resins and cellulose derivatives are also used. The sizing agents may be added directly to the stock as beater additives to produce internal or engine sizing.