**Core Concept**
The a-helix is a type of helical structure formed by hydrogen bonding between amino acids in a protein, which is a fundamental aspect of protein secondary structure. This structure is crucial for the stability and function of proteins.

**Why the Correct Answer is Right**
The a-helix is a specific type of secondary structure characterized by a spiral arrangement of amino acids, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. This arrangement allows for a regular, repeating pattern of amino acids and is a key feature of many proteins. The a-helix is a classic example of a secondary structure element, which is distinct from the primary structure (sequence of amino acids), tertiary structure (3D shape), and quaternary structure (arrangement of multiple polypeptide chains).

**Why Each Wrong Option is Incorrect**
**Option A:** Primary structure refers to the linear sequence of amino acids in a protein, which does not describe the a-helix structure.
**Option C:** Tertiary structure refers to the overall 3D shape of a single protein, which is a higher level of organization than the a-helix.
**Option D:** Quaternary structure refers to the arrangement of multiple polypeptide chains in a protein, which is not relevant to the a-helix.

**Clinical Pearl / High-Yield Fact**
The a-helix is a common secondary structure element in proteins, and understanding its characteristics is essential for predicting protein function and stability. This knowledge is critical for understanding various biological processes and designing drugs that target specific protein structures.

**✓ Correct Answer: B. Secondary structure. The a-helix is a specific type of secondary structure characterized by a spiral arrangement of amino acids, stabilized by hydrogen bonds.**