Final answer:
The helical foldings of proteins, or the alpha-helix, are stabilized by hydrogen bonds between the carbonyl oxygen and amide hydrogen within the polypeptide backbone. Thus, the option D is the correct answer.
Step-by-step explanation:
The helical foldings of proteins, known as the α-helix (alpha-helix), are stabilized mainly by hydrogen bonds that form between parts of the backbone of the polypeptide chain, specifically between the carbonyl oxygen atom (CO) of one amino acid and the amide hydrogen (NH) of another. These bonds occur in every fourth amino acid and lead to the characteristic right-handed coil of the α-helix structure. It's important to note that this stabilization refers to the protein's secondary structure, which also includes β-pleated sheets formed by similar hydrogen bonds.