**Core Concept**
Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS). It plays a crucial role in synaptic transmission, neuronal excitability, and learning and memory processes. Glutamate acts through various receptors, including NMDA, AMPA, and kainate receptors, to facilitate excitatory neurotransmission.

**Why the Correct Answer is Right**
Glutamate is the most abundant excitatory neurotransmitter in the CNS, released by presynaptic neurons and binding to postsynaptic receptors to generate an excitatory postsynaptic potential (EPSP). This leads to the depolarization of the postsynaptic neuron, making it more likely to fire an action potential. Glutamate's action is mediated by the activation of NMDA, AMPA, and kainate receptors, which are ligand-gated ion channels that allow the influx of sodium and calcium ions into the postsynaptic neuron. The NMDA receptor, in particular, is a critical component of synaptic plasticity and learning.

**Why Each Wrong Option is Incorrect**
**Option A:** Acetylcholine is an excitatory neurotransmitter, but it is primarily involved in the peripheral nervous system, particularly in the transmission of signals from the nervous system to muscles and glands.
**Option B:** Glycine is an inhibitory neurotransmitter in the CNS, acting on glycine receptors to decrease neuronal excitability and promote inhibitory neurotransmission.
**Option C:** GABA (gamma-aminobutyric acid) is also an inhibitory neurotransmitter in the CNS, acting on GABA receptors to hyperpolarize the postsynaptic neuron and reduce neuronal excitability.

**Clinical Pearl / High-Yield Fact**
Glutamate excitotoxicity is a key mechanism underlying various neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease, where excessive glutamate release and receptor activation lead to neuronal damage and death.

**✓ Correct Answer: D. Glutamate**