**Core Concept**
The management of postoperative nausea and vomiting (PONV) following squint surgery in children involves various pharmacological strategies to minimize discomfort and prevent complications. This question tests the knowledge of specific antiemetic agents used in pediatric anesthesia.

**Why the Correct Answer is Right**
The treatment of PONV in children often involves the use of ondansetron, a serotonin 5-HT3 receptor antagonist. Ondansetron works by blocking the action of serotonin at the 5-HT3 receptors in the chemoreceptor trigger zone of the brain, thereby reducing the incidence of nausea and vomiting. This mechanism is particularly effective in the context of squint surgery, where PONV is a common concern due to the proximity of the surgical site to the vomit center.

**Why Each Wrong Option is Incorrect**
**Option A:** Metoclopramide is a dopamine antagonist and 5-HT4 agonist, which can also be used to treat PONV. However, it is not the most commonly recommended agent for this indication in pediatric anesthesia due to its potential side effects, such as extrapyramidal reactions.
**Option B:** Dexamethasone is a corticosteroid that has anti-inflammatory and immunosuppressive properties. It is sometimes used in the treatment of PONV due to its ability to reduce inflammation and modulate the immune response. However, it is not a primary agent for this indication.
**Option C:** This option is missing.

**Clinical Pearl / High-Yield Fact**
When managing PONV in children, it is essential to consider the potential for serotonin 5-HT3 receptor antagonists, such as ondansetron, to be effective due to their mechanism of action. Additionally, the use of combination therapy, including corticosteroids like dexamethasone, may be beneficial in reducing the incidence of PONV.

**Correct Answer:** D.