**Core Concept**
The concept of chirality in amino acids refers to the presence of a chiral center, typically a carbon atom bonded to four different groups, leading to non-superimposable mirror images (enantiomers). This property is crucial in determining the biological activity and function of amino acids.

**Why the Correct Answer is Right**
The amino acid that lacks chirality is Glycine (Gly). Glycine has a central carbon atom bonded to three hydrogen atoms and a carboxyl group, making it the only amino acid with a non-chiral structure. This is because it lacks the asymmetric carbon atom required for chirality. As a result, Glycine exists as a single enantiomer, whereas other amino acids exist as pairs of enantiomers.

**Why Each Wrong Option is Incorrect**
* **Option A:** None of the other options (e.g., Alanine, Valine, Leucine) lack chirality, as they have a central carbon atom bonded to four different groups.
* **Option B:** Glutamic acid, like other amino acids, has a chiral center and exists as a pair of enantiomers.
* **Option C:** Aspartic acid also has a chiral center and exists as a pair of enantiomers.

**Clinical Pearl / High-Yield Fact**
Glycine's non-chiral structure is essential for its role in various biological processes, including protein synthesis, DNA replication, and neurotransmission. It is also an important precursor for the synthesis of purines and glutathione.

**Correct Answer: D. Glycine.**