**Core Concept**
Hyoscine, a scopolamine derivative, is a potent anticholinergic agent that effectively prevents motion sickness by blocking the action of acetylcholine at muscarinic receptors in the inner ear. This blockade inhibits the stimulation of the vestibular apparatus, thereby reducing the symptoms of motion sickness.

**Why the Correct Answer is Right**
Hyoscine acts by inhibiting the release of acetylcholine from the nerve terminals of the inner ear, which in turn reduces the stimulation of the vestibular apparatus. This reduction in stimulation leads to a decrease in the symptoms of motion sickness, including nausea, vomiting, and dizziness. Hyoscine's efficacy in preventing motion sickness is well-documented, making it a preferred treatment option for individuals prone to motion sickness.

**Why Each Wrong Option is Incorrect**
**Option A:** Ephedrine is a sympathomimetic agent used primarily to treat hypotension and bronchospasm. It is not effective in preventing motion sickness.

**Option B:** Nedocromil is a mast cell stabilizer used to treat allergic conditions such as asthma. It has no role in preventing motion sickness.

**Option C:** Cyproheptidine is an antihistamine with serotonin receptor antagonist properties, used primarily to treat allergic conditions. While it may have some antiemetic effects, it is not the most effective agent for preventing motion sickness.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hyoscine is available in various forms, including patches, tablets, and injectables. The patch form is particularly effective for preventing motion sickness, as it provides a sustained release of the medication over several days.

**✓ Correct Answer: D. Hyoscine**