Protein is composed of

Proteins are composed of amino acids linked together by peptide bonds. Polypeptide chains are made up of amino acids linked together by peptide bonds, while proteins are made up of 30-50 amino acids linked together. All amino acids have the same fundamental structural formula, however, their side chains differ. A carboxyl group at one end of the protein handles the acid component of the amino acid, while an amine group is at the other end. Thus, we can say that proteins are composed of amino acids.

Hence, the correct answer is option A) Amino acid.