**Core Concept:**
Intestinal obstruction is a clinical condition characterized by the blockage of the gastrointestinal tract, leading to impaired passage of luminal contents. It can be categorized into either mechanical or functional obstructions, with mechanical obstructions being more severe and potentially life-threatening.

**Why the Correct Answer is Right:**
The correct answer, **"C."**, refers to the clinical sign of "thoracic pain" as the sure sign of intestinal obstruction. This is due to the fact that when intestinal obstruction occurs, the patient experiences pain due to the back pressure of the obstructed content along the gastrointestinal tract. The pain is often described as migratory, starting in the abdomen and migrating to the thorax as the pressure in the abdomen increases, leading to diaphragmatic irritation.

**Why Each Wrong Option is Incorrect:**
A. **"A."** refers to the clinical sign of "diarrhoea" which is not a sure sign of intestinal obstruction. In fact, diarrhoea is a common presentation of intestinal obstruction, especially in cases of mechanical obstruction where the bowel contents are not being absorbed.

B. **"B."** refers to the clinical sign of "flatulence". Flatulence is a normal physiological process and not a sure sign of intestinal obstruction.

D. **"D."** refers to the clinical sign of "vomiting". While vomiting can be a presenting feature of intestinal obstruction, it is not a sure sign. Vomiting can be seen in various other gastrointestinal conditions as well.

**Clinical Pearl:**
A sure sign of intestinal obstruction is the migratory pain pattern, starting in the abdomen and migrating to the thorax. This pain pattern is a result of the pressure increase due to the obstruction, causing diaphragmatic irritation. Recognizing this sign can help in promptly diagnosing and managing the patient with intestinal obstruction.