**Core Concept**
The condition in question is likely to be intestinal obstruction in a neonate, which can be caused by various factors such as congenital anomalies, hernias, or volvulus. The key to diagnosing this condition lies in identifying the specific features that distinguish it from other causes of vomiting and abdominal distension.

**Why the Correct Answer is Right**
The correct answer is the feature that is most characteristic of intestinal obstruction. In this case, the correct answer is **multiple air-fluid levels** on a plain abdominal radiograph. This is because intestinal obstruction leads to a buildup of gas and fluid in the intestines, which can cause multiple air-fluid levels to form. The presence of multiple air-fluid levels is a strong indicator of intestinal obstruction, particularly in a neonate.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Umbilical hernia** is a common finding in neonates, but it is not a direct indicator of intestinal obstruction. While an umbilical hernia can sometimes be associated with intestinal obstruction, it is not the most essential feature in reaching the diagnosis.
* **Option B:** **Pyloric stenosis** presents with non-bilious vomiting, but it is characterized by a single, thickened pylorus on ultrasound, not multiple air-fluid levels. Pyloric stenosis is a different condition from intestinal obstruction.
* **Option C:** **Hypertrophic pyloric stenosis** is another condition that presents with non-bilious vomiting, but it is characterized by a thickened pylorus and a "doughnut" sign on ultrasound, not multiple air-fluid levels. Hypertrophic pyloric stenosis is a different condition from intestinal obstruction.

**Clinical Pearl / High-Yield Fact**
When evaluating a neonate with non-bilious vomiting and abdominal distension, it is essential to look for signs of intestinal obstruction, such as multiple air-fluid levels on a plain abdominal radiograph. This can help differentiate intestinal obstruction from other causes of vomiting and abdominal distension.

**Correct Answer:** C. Multiple air-fluid levels on a plain abdominal radiograph.