Which of these is not a lipid?

The correct option is Proteins.

To understand why proteins are not lipids, let us look at the definitions and chemical structures of both lipids and proteins:
1. Lipids are a diverse group of organic compounds that are insoluble in water but soluble in organic solvents. They include fats, oils, waxes, phospholipids, and steroids. Fats and oils are triglycerides composed of glycerol and fatty acids, while waxes are esters of long-chain fatty acids with long-chain alcohols.
2. Proteins, on the other hand, are large biomolecules (macromolecules) consisting of one or more long chains of amino acid residues. They contain nitrogen, carbon, hydrogen, oxygen, and sometimes sulfur, and they serve structural, enzymatic, and regulatory functions in cells.

Therefore, while fats, oils, and waxes are all classified as lipids due to their hydrophobic nature and chemical structure, proteins belong to an entirely different class of biological macromolecules.