What is the primary building block of carbohydrates?

Monosaccharides serve as the fundamental building blocks of carbohydrates. These simple sugars, such as glucose and fructose, consist of single sugar units that can combine in various ways to form more complex carbohydrates. For instance, two monosaccharides can link together to create disaccharides, such as sucrose, or many monosaccharides can join to create polysaccharides like starch and cellulose. The structure and properties of carbohydrates, including their energy storage and structural roles in living organisms, stem from these basic components.

In contrast, the other choices are different types of biological macromolecules: amino acids are the building blocks of proteins, fatty acids are components of lipids, and nucleotides make up nucleic acids like DNA and RNA. Each of these plays a distinct and critical role in biological processes, but they do not contribute to the structure or function of carbohydrates.