Which of the following fibers is most likely to be classified as insoluble?

• A. Inulin, a fructan found in chicory root and many prebiotic supplements.
• B. Psyllium husk, which forms a viscous gel in water.
• C. Pectin, commonly found in fruit flesh and used to make jams gel.
• D. Cellulose, commonly found in vegetable cell walls and skins of fruits.

Answer: (D)