**Core Concept**
Neutral amino acids are a group of amino acids that do not have a net charge at physiological pH, meaning their pKa values are close to physiological pH. This characteristic allows them to freely cross cell membranes.

**Why the Correct Answer is Right**
Glycine is a unique amino acid with a small side chain (H) and a pKa value close to physiological pH (around 6.0). As a result, glycine is electrically neutral and can easily cross cell membranes. This property is crucial for glycine's role as a neurotransmitter and as a precursor for purines and glutathione. Glycine's neutrality also allows it to participate in peptide bond formation without disrupting the electrical balance of the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Aspartate is a negatively charged amino acid due to its carboxyl side chain, making it an acidic amino acid rather than a neutral one.
**Option B:** Arginine is a positively charged amino acid due to its guanidinium side chain, making it a basic amino acid rather than a neutral one.
**Option D:** Histidine is a positively charged amino acid at physiological pH due to its imidazole side chain, making it a basic amino acid rather than a neutral one.

**Clinical Pearl / High-Yield Fact**
Neutral amino acids like glycine can freely cross cell membranes, making them essential for neurotransmission and peptide synthesis. This property is critical for understanding various neurological and metabolic disorders.

✓ Correct Answer: C. Glycine