**Core Concept**
The question is testing the student's knowledge of rapidly acting neurotransmitters, which are chemical messengers released by neurons to transmit signals across synapses. These neurotransmitters play a crucial role in various physiological and pathological processes, including mood regulation, movement control, and sensory perception.

**Why the Correct Answer is Right**
GABA (gamma-aminobutyric acid) is an example of a rapidly acting neurotransmitter. It is an inhibitory neurotransmitter, meaning it suppresses neuronal activity, and it acts quickly to produce its effects. GABA binds to GABA receptors, which are ligand-gated chloride channels, allowing chloride ions to flow into the neuron and hyperpolarize it, making it less likely to fire action potentials. This rapid action of GABA is crucial for regulating neuronal excitability and preventing excessive neuronal activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutamate is an example of an excitatory neurotransmitter, which is the opposite of rapidly acting GABA. Glutamate acts slowly, binding to NMDA and AMPA receptors, which depolarize the neuron and increase its excitability.
**Option B:** Acetylcholine is a slowly acting neurotransmitter, which is involved in various physiological processes, including muscle contraction and memory formation. It acts through nicotinic and muscarinic receptors, but its effects are not as rapid as those of GABA.
**Option C:** Dopamine is a neurotransmitter involved in reward processing, motor control, and mood regulation, but it is not a rapidly acting neurotransmitter like GABA. Dopamine acts through D1 and D2 receptors, but its effects are slower and more complex than those of GABA.

**Clinical Pearl / High-Yield Fact**
GABA receptors are the target of various anxiolytic and sedative medications, including benzodiazepines, barbiturates, and gabapentin. These medications work by enhancing the activity of GABA, leading to increased inhibitory neurotransmission and reduced neuronal excitability.

**Correct Answer:** D. GABA is an example for rapidly acting neurotransmitter.