**Core Concept**
Pneumoperitoneum refers to the presence of free air within the abdominal cavity, often resulting from a perforated viscus or iatrogenic trauma. Radiographic signs of pneumoperitoneum are crucial for diagnosing this condition.

**Why the Correct Answer is Right**
Rigler's sign is a characteristic X-ray finding of pneumoperitoneum, where two opposing walls of the bowel are seen to be outlined by the free air, creating a double-wall sign. This sign is a result of the air trapped between the bowel wall and the overlying peritoneum. Rigler's sign is considered a classic radiographic indicator of pneumoperitoneum, particularly when seen in conjunction with other signs such as the "cupola sign" or "pneumobilia".

**Why Each Wrong Option is Incorrect**
**Option A:** The Steeple sign is actually a feature of tracheomalacia, where the trachea appears narrow and tapered, resembling a church steeple. This sign is not related to pneumoperitoneum.
**Option C:** The Golden 'S' sign is a feature of tracheal compression by a mass, such as a thyroid goiter. This sign is not associated with pneumoperitoneum.
**Option D:** The 'Bird of prey' sign is a feature of aortic dissection, where the aorta appears to be split, resembling the beak of a bird. This sign is not related to pneumoperitoneum.

**Clinical Pearl / High-Yield Fact**
In cases of suspected pneumoperitoneum, a supine abdominal X-ray should be performed, as this view is more sensitive for detecting free air. The classic "cupola sign" of pneumoperitoneum, where a crescent-shaped collection of air is seen above the liver or spleen, is often overlooked but can be a valuable diagnostic clue.

**✓ Correct Answer: B. Rigler's sign**