**Core Concept**
The patient's presentation of fever, diarrhea, and acute onset abdominal pain, in the context of a pre-existing diagnosis of chronic ulcerative colitis, raises suspicion for a complication of inflammatory bowel disease (IBD). The erect abdominal X-ray is crucial in diagnosing the complication.

**Why the Correct Answer is Right**
The patient's symptoms and history are suggestive of toxic megacolon, a life-threatening complication of IBD. The erect abdominal X-ray shows a grossly distended colon with a diameter of >6 cm, which is a hallmark of toxic megacolon. The presence of fever and abdominal pain further supports this diagnosis. In toxic megacolon, the colon loses its normal motility and becomes severely distended, leading to potential perforation and peritonitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Pneumoperitoneum is a possible complication of toxic megacolon, but it is not the primary diagnosis based on the information provided. Pneumoperitoneum would be a secondary finding in this case.
**Option B:** Acute appendicitis is a possible differential diagnosis for abdominal pain and fever, but the presence of chronic ulcerative colitis and the erect abdominal X-ray findings make this less likely.
**Option C:** Diverticulitis is another possible cause of abdominal pain and fever, but the presence of chronic ulcerative colitis and the erect abdominal X-ray findings make this less likely.

**Clinical Pearl / High-Yield Fact**
Toxic megacolon is a medical emergency that requires prompt recognition and management. The presence of fever, abdominal pain, and a grossly distended colon on an erect abdominal X-ray are key diagnostic features.

**Correct Answer: C. Diverticulitis.**