Final answer:
Trans-fat is not one of the major classes of lipids; the major classes are triglycerides, phospholipids, and steroids.
Step-by-step explanation:
The correct answer is trans-fat. Trans-fat is not one of the three major classes of lipids, which are triglycerides, phospholipids, and steroids. Triglycerides, also known as fats and oils, are the most abundant lipids in the body and consist of a glycerol backbone attached to three fatty acid chains. Phospholipids are essential components of the plasma membrane, composed of a diglyceride with a phosphate group attached, giving them both polar and nonpolar regions. Steroids are characterized by their structure of four hydrocarbon rings and include important molecules like cholesterol. Thus, trans-fat, which is a type of fatty acid, does not represent a major class of lipids, but rather is a specific form of unsaturated fat that has been industrially transformed to have trans double bonds.