Final answer:
Vitamins are essential for metabolism to occur as they are involved in biochemical reactions in the body, and act as cofactors for enzymes in energy metabolism, particularly B vitamins.
Step-by-step explanation:
Vitamins are essential nutrients that are not synthesized by the body and are necessary for various body functions, including metabolism. They are organic compounds that are involved in many processes, such as mineral and bone metabolism, and also act as cofactors for energy metabolism. With a focus on metabolism, the B vitamins play a significant role as part of enzymes in the catalytic processes that convert substrates to end-products within the body. Therefore, the correct answer to the student's question 'Vitamins are essential for ______ to occur' is b. metabolism.