Answer: The correct answer is amino acids.
Step-by-step explanation:
Amino acids are the organic compounds which contain amine and carboxyl functional groups with a side chain that is specific to each amino acid. There are 20 amino acids.
Peptides are the short chains of amino acids that act as monomers which are linked together by peptide bonds. These covalent chemical bonds are formed when carboxyl group of one amino acid join to the amino group of another amino acid.
Polypeptides are the polymers of peptides. They are basically the chains of amino acids.
Proteins are made from one or more polypeptide molecules. The monomer for these units are amino acids.
From the above information, we get that amino acid is the only compound which are forming other chains. It acts like a monomer for all the the compounds. Hence, the correct answer is amino acids.