Answer:D. Proteins are long chains of amino acids
Explanation: Amino acids are building blocks of proteins. Amino acids are joined covalently by a peptide bond to form polypeptide chains. A peptide bond is formed by removal of elements of water from an alpha carboxyl group of an amino acid and an alpha amino group of another amino acid. These polypeptide chains formed by joining of many amino acid are packaged to form proteins.