Which structure best illustrates the secondary structure of a protein (specifically an alpha helix)?

• A. An assembly of multiple polypeptide subunits into a functional complex (quaternary structure)
• B. A specific linear sequence of amino acids connected by peptide bonds (primary structure)
• C. A right-handed helical coil stabilized by hydrogen bonds between backbone $C=O$ of residue $i$ and backbone $N-H$ of residue $i+4$
• D. A complete three-dimensional fold of a single polypeptide chain into domains (tertiary structure)

Answer: (C)