The secondary structure of a protein refers to

Proteins fold at different hierarchical levels. The term “secondary structure” is reserved for the first level of regular, repeating organisation that the polypeptide chain adopts after the simple linear sequence.

At this stage, only the backbone (i.e., the $$-N-C_{\alpha}-C=O-$$ chain) is involved, while the side-chains project outward. Hydrogen bonding between the $$C=O$$ of one peptide link and the $$N-H$$ of another stabilises two principal motifs: the $$\alpha$$-helix and the $$\beta$$-pleated sheet.

Hence, when a question asks “The secondary structure of a protein refers to …”, it is pointing to these regular backbone conformations, the most representative of which is the $$\alpha$$-helical backbone.

Option-wise:

• Option A: $$\alpha$$-helical backbone - this is precisely one of the two classical secondary structures ⇒ correct.

• Option B: Hydrophobic interactions operate mainly among side-chains and govern tertiary folding, not secondary structure ⇒ incorrect.

• Option C: The “sequence of $$\alpha$$-amino acids” is the primary structure (the covalent order) ⇒ incorrect.

• Option D: “Fixed configuration of the polypeptide backbone” is vague; secondary structure is not just any fixed configuration but specifically the regular, repeating helix or sheet patterns. ⇒ not accepted.

Therefore, the answer is:
Option A which is: $$\alpha$$-helical backbone.