**Core Concept**
In the context of blunt trauma, celiotomy (surgical exploration of the abdominal cavity) is indicated when there is evidence of intra-abdominal injury that requires surgical intervention. Peritoneal air on imaging is a significant finding that suggests intestinal perforation or other severe intra-abdominal injuries.

**Why the Correct Answer is Right**
The presence of peritoneal air on imaging, also known as pneumoperitoneum, is a critical indicator for celiotomy. This finding suggests that there is an intestinal perforation, which can lead to peritonitis and severe sepsis if left untreated. The accumulation of air in the peritoneal cavity is a sign of intestinal breach, and surgical exploration is necessary to repair the damage and prevent further complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Grade I spleen damage is typically managed conservatively with observation and monitoring, as it is considered a minor injury. Grade I splenic injuries are not an indication for celiotomy.

**Option B:** Grade II liver damage may require intervention, but it is often managed with angiographic embolization or conservative management. The decision to operate depends on the patient's hemodynamic stability and the extent of liver injury. Grade II liver damage is not an absolute indication for celiotomy.

**Clinical Pearl / High-Yield Fact**
In blunt trauma, the presence of peritoneal air on imaging is a "red flag" for celiotomy, indicating the need for urgent surgical exploration to prevent further complications.

**✓ Correct Answer: C. Peritoneal air on imaging**