## Core Concept
The question tests the knowledge of antiemetic drugs used in chemotherapy-induced nausea and vomiting (CINV). Chemotherapy-induced vomiting is a complex process involving multiple pathways and neurotransmitters, including serotonin (5-HT3) and substance P (NK1). Effective management requires targeting these pathways.

## Why the Correct Answer is Right
The correct answer, **Ondansetron**, is a selective serotonin 5-HT3 receptor antagonist. It works by blocking the action of serotonin, a natural substance that may cause nausea and vomiting. This class of drugs is highly effective in preventing acute CINV. Ondansetron and other 5-HT3 antagonists are considered first-line for preventing acute emesis associated with chemotherapy.

## Why Each Wrong Option is Incorrect
- **Option A: Metoclopramide** is a dopamine antagonist and has antiemetic properties but is less effective than 5-HT3 antagonists in chemotherapy-induced vomiting. It can be used but is not the first choice.
- **Option B: Promethazine** is a first-generation antihistamine with antiemetic properties, primarily used for its sedative effects and in treating allergic reactions. It is not the preferred agent for chemotherapy-induced vomiting.
- **Option D: Domperidone** is a peripherally selective dopamine D2 receptor antagonist used for its antiemetic and prokinetic effects. While useful in some cases of nausea and vomiting, it is not the preferred agent for chemotherapy-induced emesis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that for chemotherapy-induced nausea and vomiting, a combination of antiemetics targeting different pathways (e.g., a 5-HT3 antagonist and an NK1 antagonist like aprepitant) often provides better control than a single agent.

## Correct Answer Line
**Correct Answer: C. Ondansetron**