Answer: The correct answer for the fill in the blank is Co-enzymes.
Co-enzymes are the non proteinaceous compounds, which are necessary for the functioning of the enzymes. This means that they assist in the functioning of enzymes. Vitamins help enzymes function as co-enzymes.
For instance, Flavin adenine dinucleotide ( FAD) is a co-enzyme, which is derived from the vitamin B2, which is also called riboflavin. It helps the enzymes involved in the reduction and oxidation reactions.
Thus, co-enzymes is the right answer.