Identify what is being asked

The question asks: the $\alpha$-helix and $\beta$-pleated sheet structures of proteins belong to which level of protein structure.

Recall the levels of protein structure

• Primary structure: linear sequence of amino acids.
• Secondary structure: local folding patterns such as:
  • $\alpha$-helix
  • $\beta$-pleated sheet
• Tertiary structure: overall 3D folding of a single polypeptide chain.
• Quaternary structure: arrangement of multiple polypeptide chains.

Match the given structures

Both $\alpha$-helix and $\beta$-pleated sheet are classic examples of secondary structure of proteins.

Evaluate the options

• A: tertiary structure $\rightarrow$ Incorrect
• B: quaternary structure $\rightarrow$ Incorrect
• C: primary structure $\rightarrow$ Incorrect
• D: secondary structure $\rightarrow$ Correct

Final answer

The correct option is D.