**Core Concept**
The question is testing the student's knowledge of the management of chemotherapy-induced nausea and vomiting (CINV), a common side effect of cancer chemotherapy. The underlying principle is the use of antiemetic medications to prevent or treat CINV.

**Why the Correct Answer is Right**
The correct answer is a serotonin 5-HT3 receptor antagonist, which is a class of medications that have been shown to be highly effective in preventing CINV. These medications work by blocking the action of serotonin at the 5-HT3 receptors in the central nervous system and the vomiting center in the brain. This leads to a reduction in the vomiting reflex and the prevention of CINV.

**Why Each Wrong Option is Incorrect**
**Option A:** Metoclopramide is a dopamine antagonist that can be used to prevent CINV, but it is not the first-line treatment and has a lower efficacy compared to 5-HT3 receptor antagonists.
**Option B:** Ondansetron is a 5-HT3 receptor antagonist, but it is not the most commonly used medication for CINV.
**Option C:** Dexamethasone is a corticosteroid that can be used to prevent CINV, but it is not the first-line treatment and is often used in combination with other medications.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the first-line treatment for CINV is "5-HT3", which stands for serotonin 5-HT3 receptor antagonists. These medications are highly effective in preventing CINV and should be used as the first-line treatment.

**Correct Answer:** C.