The correct option is A H-bond
Proteins are the polypeptides of amino acids. Proteins have a different structure which is because of a different arrangement of amino acids. The primary structure of the protein has covalent connections in the linear sequence in which amino acids are bonded together with peptide bonds. The folding of the linear polypeptide chain into a specific structure that has the alpha helix is called the secondary structure.
In the secondary structure, the hydrogen bonds are also formed. It stabilises the complex structure of the secondary structure of the protein.
Thus, hydrogen bond is responsible for the stacking of α-helix structure of protein. α-helix is helical in shape because of hydrogen bonding between >C=O and -NH group.
Hence, option (a) is correct.