**Core Concept**
The a-helix is a fundamental secondary structure of proteins, characterized by a spiral conformation where the peptide backbone forms a helix. This structure is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away.

**Why the Correct Answer is Right**
The a-helix is the most common secondary structure of proteins, accounting for approximately 40% of all protein structures. It is stabilized by the hydrogen bonding between the carbonyl oxygen (C=O) and the amide hydrogen (N-H) of adjacent amino acids, four residues apart. This specific geometry allows for the efficient packing of amino acids and the formation of a compact, helical structure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the a-helix is not a coiled structure, but rather a spiral or helical one.
**Option B:** This option is incorrect because the beta-sheet is a different secondary structure of proteins, characterized by a layered or sheet-like arrangement of amino acids.

**Clinical Pearl / High-Yield Fact**
The a-helix is a critical component of many enzymes, antibodies, and structural proteins, and its stability is essential for maintaining protein function and structure.

**Correct Answer:** C.