Solution:

• Vitamins mainly function as coenzymes. Coenzymes are organic molecules that help enzymes to work.
• Enzymes are proteins that speed up chemical reactions in the body.
• Vitamins are essential for many metabolic processes in the body, such as energy production, cell growth and repair, and immune function.
• Nucleic acids (DNA and RNA) are the genetic material of all living cells. Substrates are the molecules that enzymes convert into products. Nucleotides are the building blocks of nucleic acids.
• Therefore, the answer is A.