**Core Concept**
The 5HT3 antagonists are a class of medications that selectively block the action of serotonin at the 5HT3 receptor, a subtype of serotonin receptor found in the central and peripheral nervous system. These receptors are involved in various physiological processes, including nausea, vomiting, and pain modulation.

**Why the Correct Answer is Right**
The correct answer is a medication that is specifically used to prevent nausea and vomiting caused by chemotherapy, radiation therapy, and surgery. The mechanism of action of 5HT3 antagonists involves blocking the action of serotonin at the 5HT3 receptor, which is located in the vagus nerve terminals and in the central nervous system. This blockade prevents the stimulation of the vomiting center in the brain, ultimately reducing the incidence of nausea and vomiting.

**Why Each Wrong Option is Incorrect**
**Option A:** This medication is actually a 5HT2 antagonist, which has different clinical applications such as treating schizophrenia and bipolar disorder.

**Option B:** This medication is a GABA receptor agonist, used to treat anxiety disorders and insomnia, and has no effect on 5HT3 receptors.

**Option C:** This medication is a selective serotonin reuptake inhibitor (SSRI), which increases the levels of serotonin in the synaptic cleft, and has no direct effect on 5HT3 receptors.

**Clinical Pearl / High-Yield Fact**
The 5HT3 antagonists are often used in combination with other antiemetic medications to provide optimal control of nausea and vomiting in patients undergoing chemotherapy or radiation therapy.

**Correct Answer:** C. Ondansetron