**Core Concept**
Motion sickness is a common condition caused by conflicting signals between the sensory inputs from the visual, vestibular, and proprioceptive systems, leading to nausea and vomiting. The drug of choice for prophylaxis of motion sickness should effectively counteract these conflicting signals and alleviate symptoms.

**Why the Correct Answer is Right**
Scopolamine, a tropane alkaloid, is the drug of choice for prophylaxis of motion sickness. It works by blocking the action of acetylcholine at muscarinic receptors in the central nervous system, thereby reducing the conflicting signals from the sensory systems. This results in decreased stimulation of the vomiting center and alleviation of symptoms. Scopolamine also has a sedative effect, which can further help to reduce the severity of motion sickness.

**Why Each Wrong Option is Incorrect**
**Option A:** Metoclopramide is a dopamine antagonist that is used to treat nausea and vomiting, but it is not effective for prophylaxis of motion sickness.
**Option B:** Ondansetron is a serotonin 5-HT3 receptor antagonist that is used to prevent nausea and vomiting caused by chemotherapy and radiation therapy, but it is not the first-line treatment for motion sickness.
**Option C:** Dimenhydrinate is an antihistamine that is used to treat motion sickness, but it is less effective than scopolamine and has more side effects.

**Clinical Pearl / High-Yield Fact**
Scopolamine patches are available for the prophylaxis of motion sickness and can be applied behind the ear 4-6 hours before traveling. This can provide effective protection against motion sickness for up to 3 days.

**Correct Answer: C. Dimenhydrinate is an antihistamine that is used to treat motion sickness, but it is less effective than scopolamine and has more side effects.