Question

as more amino acid monomers are linked together, a long polypeptide chain forms. the polypeptide chain eventually becomes a protein.
select the polypeptide.

Explanation:

A polypeptide is a chain of amino acids linked by peptide bonds. The first structure has two amino acids (dipeptide), the second is a single amino acid, and the third has multiple amino acids linked together forming a longer chain which is characteristic of a polypeptide.

Answer:

The third structure (the longest chain of amino - acid - like units linked by peptide - like bonds) is the polypeptide.