**Core Concept**
The question tests the understanding of amino acid stability at physiological pH, which is around 7.4. **Amino acids** have different stability profiles due to their unique side chains and **pKa values**.

**Why the Correct Answer is Right**
The most stable amino acid at physiological pH is one that has a **pKa value** close to physiological pH, allowing it to exist in a **zwitterion form**, which is more stable.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because its side chain does not contribute to stability at physiological pH.
**Option B:** Incorrect as it is not the most stable form.
**Option D:** Incorrect due to its instability.

**Clinical Pearl / High-Yield Fact**
The stability of amino acids is crucial in understanding **protein structure** and function.

**Correct Answer:** C. Glycine