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Amino acids are the fundamental chemical units that compose proteins. They are organic compounds that contain both an amine group and a carboxyl group, along with a distinctive side chain for each type of amino acid. There are 20 different amino acids that can combine in various arrangements to form proteins, which are crucial for numerous biological functions such as building tissues, producing enzymes, and supporting immune function.

Other options, like fatty acids, are the building blocks of lipids and are not involved in the structure of proteins. Monosaccharides are the basic units of carbohydrates, while peptide bonds are the specific covalent bonds that link amino acids together to form a protein. While the existence of peptide bonds is essential to protein structure, they do not represent the individual units that constitute proteins. Thus, the role of amino acids as the basic building blocks is why this option is the correct answer.