Final answer:
Water (A) is the solvent commonly used to dissolve cotton fibers due to the high affinity between water and the hydroxyl groups present in cellulose.
Step-by-step explanation:
The solvent that is commonly used to dissolve cotton fibers is water (A). Cotton is composed mainly of cellulose, which is a complex carbohydrate and insoluble in organic solvents such as acetone, ethanol, and hexane. However, cotton fibers have a high affinity for water molecules due to the presence of hydroxyl groups in cellulose, making water an effective solvent for dissolving cotton fibers.