In the context of protein structure, what best describes the tertiary structure of a protein?

• A. The linear sequence of amino acids in a polypeptide chain linked by peptide bonds.
• B. Local, repetitive backbone conformations such as $\alpha$-helices and $\beta$-sheets stabilized primarily by backbone hydrogen bonding.
• C. The overall three-dimensional folding of a single polypeptide chain formed by interactions among side chains (e.g., hydrophobic interactions, hydrogen bonds, ionic interactions, and disulfide bonds).
• D. The arrangement and interactions of multiple polypeptide subunits to form a functional protein complex.

Answer: (C)