Glucose can exist in both linear and cyclic forms. Which structural feature allows glucose to form a cyclic structure?

• A. The presence of a phosphate group that can react with the hydroxyl group.
• B. The presence of a carboxyl group that can react with the ketone group.
• C. The presence of a methyl group that can react with the aldehyde group.
• D. The presence of a hydroxyl group that can react with the aldehyde group.

Answer: (D)