Final answer:
All of the classes of lipids mentioned, including phospholipids, sphingolipids, and cholesterol, are present in biomembranes. Phospholipids form the membrane structure, while sphingolipids and cholesterol contribute to membrane stability and fluidity.
Step-by-step explanation:
The classes of lipids that are present in biomembranes include phospholipids, sphingolipids, and cholesterol. Therefore, the correct answer to the question is (d) all of the above. Phospholipids form the fundamental structure of the membrane, creating a bilayer that serves as a barrier and matrix for other molecules. Sphingolipids are also important components of the membrane, contributing to its stability and functionality. Cholesterol, categorized as a steroid, is also integral to plasma membranes, where it is embedded within the tail bilayer, influencing fluidity and mechanical properties of the membrane.