**Core Concept**
The question is testing the student's understanding of the optical isomerism and chirality in amino acids. Optical isomerism occurs when a molecule has a non-superimposable mirror image, resulting in two enantiomers that rotate plane-polarized light in opposite directions. This property is crucial in understanding the behavior of amino acids in biological systems.

**Why the Correct Answer is Right**
Glycine is the only amino acid that is optically inactive because it lacks a chiral center. In glycine, the side chain is a hydrogen atom, which does not contribute to the molecule's chirality. As a result, glycine exists as a racemic mixture of two enantiomers, but it does not exhibit optical activity due to the lack of a net optical rotation. This unique property of glycine is due to its structure, which does not allow for the formation of a chiral center.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we'll skip it.
* **Option B:** This option is also not provided, so we'll skip it.
* **Option C:** This option is also not provided, so we'll skip it.
* **Option D:** This option is also not provided, so we'll skip it.

**Clinical Pearl / High-Yield Fact**
Glycine is the smallest and simplest amino acid, and it plays a crucial role in various biological processes, including the synthesis of purines, glutathione, and creatine. Its unique structure and lack of chirality make it an important molecule in biochemistry.

**Correct Answer:** D. Glycine.