**Core Concept**
The question tests the knowledge of **inhibitory neurotransmitters** in the central nervous system (CNS), specifically in the spinal cord. **Glycine** and **gamma-aminobutyric acid (GABA)** are key inhibitory neurotransmitters in the CNS.

**Why the Correct Answer is Right**
Glycine is an inhibitory neurotransmitter in the spinal cord, acting on **glycine receptors**, which are ligand-gated chloride channels. This action results in hyperpolarization of the neuron, reducing the likelihood of action potential firing.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the specific neurotransmitter is not identified.
**Option B:** Incorrect as it is not provided.
**Option C:** Incorrect as it is not specified.
**Option D:** Incorrect because it is missing.

**Clinical Pearl / High-Yield Fact**
Remember that **glycine** is a major inhibitory neurotransmitter in the spinal cord, while **GABA** plays a similar role in higher CNS centers.

**Correct Answer:** D. Glycine.