Which of the following is a type of lipid?

A steroid is a type of lipid because it is characterized by a structure that includes four fused carbon rings and is hydrophobic, meaning it does not mix well with water. Steroids play key roles in the body, such as functioning as hormones (like testosterone and estrogen) and components of cell membranes (like cholesterol). They are part of a broader category of lipids that includes fats, oils, and phospholipids, all of which share the property of being largely non-polar and insoluble in water.

In the context of the other options, carbohydrates are primarily energy sources and structural components of cells, amino acids are the building blocks of proteins, and nucleotides are the basic units of nucleic acids like DNA and RNA. These categories have distinct functions and structures that do not class them as lipids. Therefore, the identification of steroids as lipids is in alignment with their chemical structure and biological functions.