**Core Concept**
The a-helix is a type of secondary structure in proteins, characterized by a spiral conformation where the polypeptide chain is twisted into 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 correct answer is related to the properties of the a-helix structure. The a-helix is known for its rigid and stable conformation, which allows it to maintain its shape and function in proteins. The hydrogen bonding between residues, as well as the hydrophobic interactions between non-polar side chains, contribute to the stability of the a-helix. The regular spacing of the helix, approximately 5.4 Å between residues, allows for efficient packing of the protein backbone.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the a-helix is characterized by a regular spacing of approximately 5.4 Å between residues, not 3.6 Å.
**Option B:** This option is incorrect because the a-helix is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away, not six residues away.
**Option C:** This option is incorrect because the a-helix is indeed a type of secondary structure in proteins, characterized by a spiral conformation.

**Clinical Pearl / High-Yield Fact**
The a-helix is a common secondary structure found in many proteins, including enzymes, receptors, and structural proteins. Understanding the properties and stability of the a-helix is essential for understanding protein folding and function.

**Correct Answer: A. The a-helix has a regular spacing of approximately 5.4 Å between residues.**