Which class of biological molecules includes enzymes?

Carbohydrates are primarily energy sources and structural components in cells, such as starch and cellulose. While some carbohydrates can play roles in cellular signaling, they do not function as enzymes. Enzymatic activity is specifically a characteristic of proteins, making this option incorrect.

Vitamins are organic compounds that organisms require in small quantities for various biochemical functions. Although some vitamins can act as coenzymes or cofactors in enzyme reactions, they themselves are not enzymes. This distinction means that vitamins cannot be classified as enzymes, thereby rendering this choice incorrect.

Lipids serve as energy storage molecules, structural components of cell membranes, and signaling molecules. While they play many vital roles in biological systems, they do not possess the catalytic properties associated with enzymes. Thus, lipids are not classified as enzymes, making this option incorrect.