Which of the following substances is classified as a lipid?

Fatty acids are classified as lipids due to their chemical structure and properties. Lipids are a diverse group of hydrophobic molecules that are primarily composed of carbon and hydrogen. Fatty acids consist of long hydrocarbon chains with a carboxyl group (-COOH) at one end. They can be saturated or unsaturated, depending on the presence of double bonds between carbon atoms.

Lipids play various roles in biological systems, including energy storage, structural components of cell membranes, and signaling molecules. Fatty acids serve as key building blocks for more complex lipids, such as triglycerides and phospholipids, which are essential for forming cellular membranes and storing energy in living organisms.

Other substances like insulin, glucose, and amino acids belong to different categories of biomolecules. Insulin is a peptide hormone, glucose is a simple sugar (carbohydrate), and amino acids are the building blocks of proteins. Each of these plays significant roles in metabolism and physiological processes but does not fit the classification of lipids.