**Core Concept**
The formation of an intraabdominal abscess occurs when a collection of pus accumulates within the abdominal cavity, often as a result of a bacterial infection. This can arise from a variety of sources, including perforated viscera, surgical sites, or contiguous spread from adjacent organs.

**Why the Correct Answer is Right**
The commonest sites of intraabdominal abscess formation are typically related to the areas with the highest concentration of lymphatic and blood vessels. The liver and spleen are common sites due to their rich vascular supply and proximity to the gastrointestinal tract. The pelvis is another common location, particularly in cases of diverticulitis or colorectal cancer. The formation of an abscess in these areas often requires surgical drainage or percutaneous intervention to prevent further complications.

**Why Each Wrong Option is Incorrect**
**Option A:** While the peritoneal cavity is indeed a potential site for abscess formation, it is not the most common location. The peritoneum is a serous membrane that lines the abdominal cavity and covers the abdominal organs, but it is not typically where abscesses accumulate.

**Option B:** The subphrenic space is a potential location for abscess formation, particularly in cases of liver or spleen disease. However, it is not the most common site compared to other intraabdominal areas.

**Option C:** The retroperitoneum is a complex space that contains lymph nodes, blood vessels, and nerves. While abscesses can form in this area, it is not the most common location due to its relatively low concentration of lymphatic and blood vessels.

**Clinical Pearl / High-Yield Fact**
Remember the "PALS" mnemonic to help recall the common sites of intraabdominal abscess formation: Pelvis, Ascending colon, Liver, and Spleen.

**Correct Answer: C. Pelvis**