**Core Concept**
Baclofen is a central nervous system depressant, primarily used as an antispastic agent. It acts on the GABA_B receptors in the spinal cord and brain, leading to muscle relaxation and reduced spasticity.

**Why the Correct Answer is Right**
Baclofen is a GABA_B receptor agonist, which means it binds to and activates these receptors. This activation leads to an increase in potassium conductance, resulting in hyperpolarization of the postsynaptic neuron and a decrease in the release of excitatory neurotransmitters. As a result, muscle tone is reduced, and spasticity is alleviated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because baclofen does not primarily act as a GABA_A receptor agonist. While GABA_A receptors are involved in the regulation of muscle tone, baclofen's mechanism of action is more specific to GABA_B receptors.
**Option B:** This option is incorrect because baclofen does not primarily act as a dopamine receptor agonist. Dopamine receptors are involved in various motor control pathways, but baclofen's primary mechanism of action is through GABA_B receptors.
**Option C:** This option is incorrect because baclofen does not primarily act as a benzodiazepine. Benzodiazepines are a class of medications that enhance the activity of GABA_A receptors, but baclofen's mechanism of action is more specific to GABA_B receptors.

**Clinical Pearl / High-Yield Fact**
Baclofen is unique in that it can be administered intrathecally for the treatment of severe spasticity. This route of administration allows for a high concentration of the medication to be delivered directly to the spinal cord, making it an effective treatment option for patients with severe spasticity.

**Correct Answer: D. GABA_B receptor agonist**