**Core Concept**
The underlying principle being tested is the diagnosis of a neonate with suspected intestinal obstruction and perforation, likely due to **volvulus neonatorum**, which is a twisting of the intestine that can lead to ischemia and necrosis. This condition requires immediate diagnosis and intervention.

**Why the Correct Answer is Right**
Given the clinical presentation of fever, lethargy, abdominal distension, vomiting, and constipation in a neonate, along with the suspicion of **volvulus neonatorum** with perforation, an urgent and accurate diagnostic tool is needed. The best investigation in such a scenario would typically involve imaging that can quickly and accurately confirm the presence of intestinal obstruction, volvulus, and potential perforation.

**Why Each Wrong Option is Incorrect**
**Option A:** Would not be the best initial choice for diagnosing volvulus or perforation.
**Option B:** Might not provide sufficient detail for intestinal perforation or volvulus.
**Option C:** Could be useful but might not be as immediate or definitive as needed for a neonate with suspected perforation.

**Clinical Pearl / High-Yield Fact**
In cases of suspected intestinal perforation or volvulus in neonates, timely diagnosis is crucial. Imaging studies, particularly those that can quickly identify free air in the abdomen (indicative of perforation) or signs of volvulus, are critical.

**Correct Answer:** D. Abdominal X-ray.