**Core Concept**
The question is testing the effectiveness of medications in smoking cessation, a critical aspect of addiction medicine. Smoking cessation is a complex process involving nicotine withdrawal, and various pharmacological agents have been studied for their potential in aiding this process.

**Why the Correct Answer is Right**
Varenicline, a partial agonist of the nicotinic acetylcholine receptor, has been found to be effective in smoking cessation. It works by activating the nicotinic receptors in the brain, reducing the craving for nicotine and the pleasurable effects of smoking. This mechanism is thought to be responsible for its efficacy in helping individuals quit smoking. Varenicline also reduces the rewarding effects of nicotine, making it easier to abstain from smoking.

**Why Each Wrong Option is Incorrect**
* **Option A:** Bupropion, an antidepressant, has been used off-label for smoking cessation. However, it is not as effective as varenicline in reducing cravings and the pleasurable effects of smoking.
* **Option B:** Nortriptyline, a tricyclic antidepressant, has been studied for smoking cessation but is not as effective as varenicline. Its side effect profile also makes it less desirable.
* **Option C:** Clonidine, an alpha-2 adrenergic agonist, has been used for opioid withdrawal but not specifically for smoking cessation.

**Clinical Pearl / High-Yield Fact**
Varenicline is a Food and Drug Administration (FDA)-approved medication for smoking cessation and has been shown to increase the chances of quitting smoking by 50% compared to placebo.

**Correct Answer:** C. Varenicline.