Recall the levels of protein structure

Protein structure is commonly described at four levels:

1. Primary structure: the sequence of $\alpha$-amino acids in the polypeptide chain.
2. Secondary structure: local folding patterns of the polypeptide backbone such as $\alpha$-helix and $\beta$-pleated sheet.
3. Tertiary structure: overall 3D folding of a single polypeptide chain, often stabilized by hydrophobic interactions, hydrogen bonds, disulfide bonds, etc.
4. Quaternary structure: arrangement of multiple polypeptide subunits.

Evaluate each option

Option A: $\alpha$-helical backbone

• The $\alpha$-helix is a classic example of secondary structure.
• So this option correctly refers to secondary structure.

Option B: hydrophobic interactions

• Hydrophobic interactions mainly help stabilize the tertiary structure of proteins.
• Hence, this is not the best description of secondary structure.

Option C: sequence of $\alpha$-amino acids

• This defines the primary structure of a protein.
• So this is incorrect.

Option D: fixed configuration of the polypeptide backbone

• This statement is somewhat broad, but in standard biomolecule terminology, secondary structure specifically refers to regular local arrangements like $\alpha$-helix or $\beta$-sheet.
• Thus, this is not the standard single-correct answer here.

Conclusion

The best answer is: $\boxed{\text{A}}$

Comparison with stored correct answer

Stored correct answer = A

My derived answer = A

Therefore, they agree.