**Core Concept**
Intestinal obstruction is a mechanical or functional obstruction of the intestine that can be caused by various factors, including adhesions, hernias, tumors, or foreign bodies. The clinical presentation and diagnostic features of intestinal obstruction are crucial for early recognition and management.

**Why the Correct Answer is Right**
The correct answer, **ab**, represents two key clinical features of intestinal obstruction: abdominal distension and localized tenderness. Abdominal distension occurs due to accumulation of gas and fluid within the obstructed bowel segment, leading to increased abdominal pressure. Localized tenderness is often present over the site of obstruction, resulting from irritation of the peritoneum by the obstructed bowel contents. These symptoms are commonly seen in patients with mechanical intestinal obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Abdominal distension is indeed a feature of intestinal obstruction, but fluid level in X-ray is not the most sensitive or specific indicator. A fluid level in X-ray may not always be present, especially in early stages of obstruction or in cases of low-grade obstruction.

**Option B:** Vomiting is a common symptom of intestinal obstruction, but it is not as specific or sensitive as abdominal distension and localized tenderness. Vomiting can occur due to various reasons, including gastric outlet obstruction or other conditions.

**Option C:** Fluid level in X-ray > 4 is not a reliable or specific indicator of intestinal obstruction. The presence of a fluid level in X-ray can be seen in various conditions, including gastric dilatation or other non-obstructive causes.

**Option E:** Diarrhea is not a typical feature of intestinal obstruction, especially in mechanical obstruction. Diarrhea may occur in cases of pseudo-obstruction or other functional disorders.

**Clinical Pearl / High-Yield Fact**
In cases of suspected intestinal obstruction, it is essential to consider the clinical presentation and diagnostic features, such as abdominal distension and localized tenderness, in conjunction with imaging studies and laboratory investigations.

**✓ Correct Answer: D. ab. Abdominal distension and localized tenderness are key clinical features of intestinal obstruction.**