Which of the following correctly defines lipids?

Lipids are correctly defined as biomolecules that are primarily insoluble in water and consist of carbon, hydrogen, and oxygen atoms. This hydrophobic nature allows lipids to serve various essential functions in biological systems, including energy storage, structural components of cell membranes, and signaling molecules.

Lipids encompass a wide range of substances such as fats, oils, steroids, and phospholipids, which all share the common characteristic of being nonpolar and, therefore, not dissolvable in water. The composition of carbon, hydrogen, and oxygen is significant, but unlike carbohydrates, lipids have a much lower proportion of oxygen relative to hydrogen. This results in their higher energy content.

The other choices are related to different biochemical categories or characteristics. For instance, some options reference solubility in water or components of proteins, which are more fitting descriptions for other biomolecules such as carbohydrates or proteins. Additionally, stating that lipids are always solid at room temperature does not capture the full diversity of lipids, as some, like oils, may be liquid under those conditions. Similarly, lipids are not composed of amino acids, which are the building blocks of proteins. The correct answer succinctly captures the essential features that define lipids in biological contexts.