**Core Concept**
Motion sickness is a common problem caused by conflicting signals between the senses, leading to nausea, vomiting, and dizziness. To prevent motion sickness, medications that affect the vestibular system or the central nervous system can be used.

**Why the Correct Answer is Right**
Scopolamine, a muscarinic antagonist, is the most effective medication for preventing motion sickness. It works by blocking the action of acetylcholine at muscarinic receptors in the inner ear, thereby reducing the conflicting signals between the senses. This leads to a reduction in the symptoms of motion sickness. Scopolamine can be administered through transdermal patches or oral tablets.

**Why Each Wrong Option is Incorrect**
**Option A:** Metoclopramide is a dopamine antagonist that is primarily used to treat nausea and vomiting caused by chemotherapy, radiation therapy, and gastrointestinal disorders. It is not effective in preventing motion sickness.

**Option B:** Ondansetron is a serotonin 5-HT3 receptor antagonist that is used to prevent nausea and vomiting caused by chemotherapy, radiation therapy, and surgery. It is not effective in preventing motion sickness.

**Option C:** Promethazine is an antihistamine that is sometimes used to treat motion sickness. However, it is less effective than scopolamine and can cause drowsiness as a side effect.

**Clinical Pearl / High-Yield Fact**
Scopolamine is the most effective medication for preventing motion sickness, but it can cause side effects such as dry mouth, blurred vision, and dizziness. It is essential to weigh the benefits and risks of scopolamine before using it to prevent motion sickness.

**Correct Answer:** C. Promethazine.