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:

1. Recall the definition of a polypeptide: a chain of multiple amino acids linked by peptide bonds ($\ce{-CO - NH -}$).
2. Analyze each option:

• The first structure has 2 amino acid residues (2 peptide - like linkages? No, it has one peptide bond, so it's a dipeptide (2 amino acids).
• The second structure has multiple peptide bonds (more than 2 amino acid residues linked), so it is a polypeptide (a long chain of amino acids).
• The third structure has 1 amino acid residue (no peptide bonds between multiple amino acids), so it's a single amino acid or a monomer, not a polypeptide.

Answer:

The second structure (the middle one with multiple peptide bonds and amino acid residues, shown as $\ce{...-N-C(CH_2C(CH_3)_2H)-C(=O)-N-C(HH)-C(=O)-N-C(CH_2C(=O)NH_2)-C(=O)-N-C(CH_2OH)-C(=O)-...}$)