Final answer:
The poly-proline II helix is primarily composed of proline residues, which due to their unique ring-like structure, impart significant structural roles in proteins.
Step-by-step explanation:
The poly-proline II helix is mainly made up of C) Proline residues. Proline is unique among the standard amino acids in that its side chain is bonded to the nitrogen of its amino group, forming a ring-like structure. This makes proline an exception in the structure of amino acids and gives it distinctive conformational rigidity. It influences the folding and function of proteins that incorporate proline due to the constraints this structure imposes on the flexibility of the polypeptide chain. The poly-proline II helix does not have the regular hydrogen bond pattern observed in α-helices and β-pleated sheets, but it still plays an important structural role in proteins.