Enzymes, vitamins and hormones can be classified into a single category of biological chemicals, because all of these:

Enzymes, vitamins, and hormones can be classified into a single category of biological chemicals because all of these help in regulating metabolism. Enzymes are biological catalysts that speed up chemical reactions, vitamins are essential nutrients that are required for various metabolic processes, and hormones are chemical messengers that regulate various physiological processes. So, the correct answer is option D. Notice that the other options are not correct. While enzymes, vitamins, and hormones may enhance oxidative metabolism, they are not all conjugated proteins. Additionally, not all of these molecules are exclusively synthesized in the body of a living organism.