**Core Concept**
The question is testing the student's knowledge of the ionization state of amino acids at different pH levels. Amino acids can exist in various forms due to the ionization of their amino and carboxyl groups, which affects their biological activity.

**Why the Correct Answer is Right**
The correct answer is a basic amino acid that remains uncharged at neutral pH. This is because it lacks a side chain that would donate a proton (H+), allowing it to maintain a neutral charge at physiological pH. At neutral pH, the amino group (-NH2) of the amino acid is protonated, and the carboxyl group (-COOH) is deprotonated, resulting in a stable, uncharged molecule.

**Why Each Wrong Option is Incorrect**
* **Option A:** This amino acid is acidic and will be negatively charged at neutral pH, making it inactive.
* **Option B:** This amino acid is also acidic and will be negatively charged at neutral pH, making it inactive.
* **Option D:** This amino acid is non-essential and will have a positive charge at neutral pH due to the protonation of its amino group, making it inactive.

**Clinical Pearl / High-Yield Fact**
At physiological pH, amino acids can be broadly classified into acidic, basic, and neutral categories based on their ionization state. Understanding the ionization state of amino acids is crucial for predicting their biological activity and function in the body.

**Correct Answer:** C.