Proteins are primarily composed of which of the following?

Proteins are primarily composed of amino acids, which are the building blocks that link together in specific sequences to form proteins. Each protein has a unique structure and function, determined by the particular sequence and arrangement of the amino acids it contains. There are 20 different amino acids, and the combination of these amino acids can produce an immense variety of proteins that play vital roles in biological processes, including tissue building, enzyme function, and immune response.

The other options do not directly pertain to the composition of proteins. Fatty acids are the building blocks of lipids, not proteins. Sugars, or carbohydrates, are also distinct macromolecules made up of different units called monosaccharides. Vitamins are organic compounds that are essential for various metabolic processes but do not serve as the building blocks for proteins. Hence, amino acids are the correct answer as they are fundamental to the structure and function of proteins.