**Core Concept**
Varenicline, a partial agonist at the nicotinic acetylcholine receptor (nAChR), is used for smoking cessation by reducing cravings and the pleasurable effects of smoking.

**Why the Correct Answer is Right**
Varenicline acts by binding to the α4β2 subunit of the nAChR, which is the primary receptor subtype involved in nicotine's rewarding effects. By occupying these receptors, varenicline reduces the release of dopamine, a neurotransmitter associated with pleasure and reward, thereby decreasing the motivation to smoke. This mechanism is distinct from traditional nicotine replacement therapy (NRT) and bupropion, which only manage withdrawal symptoms.

**Why Each Wrong Option is Incorrect**
* **Option B:** Acamprosate is a GABA_A receptor antagonist used primarily to treat alcohol dependence, not smoking cessation.
* **Option C:** Nalmefene is an opioid receptor antagonist used to treat alcohol dependence and opioid overdose, not smoking cessation.
* **Option D:** Gabapentin is an anticonvulsant used to treat epilepsy, neuropathic pain, and anxiety disorders, but it is not indicated for smoking cessation.

**Clinical Pearl / High-Yield Fact**
Varenicline's efficacy in smoking cessation is highest when combined with counseling and behavioral support, highlighting the importance of a comprehensive treatment approach.

**✓ Correct Answer: A. Varenicline**