**Core Concept**
Excitatory neurotransmitters are chemical messengers released by neurons that stimulate the postsynaptic neuron, increasing the likelihood of action potential generation. They play a crucial role in various physiological processes, including muscle contraction, learning, and memory.

**Why the Correct Answer is Right**
Glutamine is an excitatory neurotransmitter that acts on the N-methyl-D-aspartate (NMDA) receptors and to a lesser extent on AMPA receptors. When glutamine binds to these receptors, it allows an influx of sodium and calcium ions into the postsynaptic neuron, depolarizing the membrane and increasing the likelihood of action potential generation. This excitatory effect is essential for learning and memory, as well as muscle contraction and movement.

**Why Each Wrong Option is Incorrect**
**Option A:** Acetylcholine is actually a neurotransmitter that can have both excitatory and inhibitory effects, depending on the type of receptor it binds to. It is primarily involved in muscle contraction and the transmission of nerve impulses at the neuromuscular junction.

**Option B:** Glycine is an inhibitory neurotransmitter that acts on glycine receptors, which are chloride channels. When glycine binds to these receptors, it allows an influx of chloride ions into the postsynaptic neuron, hyperpolarizing the membrane and decreasing the likelihood of action potential generation.

**Option C:** GABA (Gamma-Aminobutyric Acid) is also an inhibitory neurotransmitter that acts on GABA receptors, which are chloride channels. Similar to glycine, GABA binding to these receptors allows an influx of chloride ions, hyperpolarizing the membrane and decreasing the likelihood of action potential generation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that excitatory and inhibitory neurotransmitters have opposite effects on the postsynaptic neuron, with excitatory neurotransmitters increasing the likelihood of action potential generation and inhibitory neurotransmitters decreasing it. This fundamental concept is crucial for understanding various neurological disorders and the effects of different medications on the nervous system.

**✓ Correct Answer: D. Glutamine Glutamine is an excitatory neurotransmitter that acts on NMDA and AMPA receptors to increase the likelihood of action potential generation.**