Which of the following fat soluble vitamin has been  identified to function as a coenzyme?: (A) Vitamin A, (B) Vitamin K, (C) Vitamin E, (D) Vitamin D
Answer is B. Most of the water soluble vitamins exert the functions through their respective coenzymes while only one fat soluble vitamin (K) has been identified to function as a coenzyme.
Reference: Satyanarayana- Biochemistry, 3rd edition, pg-117