**Core Concept**
The α-helix is a fundamental secondary structure of proteins, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. This structure is crucial for protein stability and function.

**Why the Correct Answer is Right**
The amino acid proline is an exception to the α-helix formation due to its unique side chain, which forms a ring structure. This ring structure prevents the peptide bond from adopting the typical α-helix conformation, as the carbonyl oxygen is not available for hydrogen bonding. As a result, proline is unable to participate in the α-helix formation, making it a key exception in protein secondary structure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine is a small amino acid that can participate in α-helix formation, although it may be over-represented in flexible regions of a protein.
* **Option B:** There is no specific amino acid that is commonly known to be unable to participate in α-helix formation besides proline.
* **Option C:** Histidine, like other amino acids, can participate in α-helix formation, although its side chain can influence protein stability and function.

**Clinical Pearl / High-Yield Fact**
Proline's unique structure can disrupt protein secondary structures, leading to changes in protein function and stability. This is particularly relevant in diseases such as Alzheimer's, where proline-rich sequences have been implicated in amyloid beta aggregation.

**Correct Answer:** D. Proline.