**Core Concept**
The question is testing knowledge of the common sites for abdominal abscesses following laparotomy, which is a post-surgical complication. Laparotomy is a surgical incision into the abdominal cavity, and abdominal abscesses are collections of pus that can form in the abdominal cavity due to infection.

**Why the Correct Answer is Right**
The most common site for infra-abdominal abscess following laparotomy is the pelvis. This is because the pelvis is a dependent area, meaning it is lowest in the abdominal cavity, and therefore, any fluid collections or abscesses tend to accumulate in this area. The pelvis also has a rich blood supply and a complex anatomy, which can contribute to the formation of abscesses.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the left subphrenic space is a common site for abscesses following upper abdominal surgery, but it is not the most common site for infra-abdominal abscesses following laparotomy.
* **Option B:** This option is incorrect because the right subphrenic space is also a common site for abscesses following upper abdominal surgery, but it is not the most common site for infra-abdominal abscesses following laparotomy.
* **Option D:** This option is incorrect because the right iliac fossa is a common site for appendicitis and abscesses, but it is not the most common site for infra-abdominal abscesses following laparotomy.

**Clinical Pearl / High-Yield Fact**
The pelvis is a common site for abscesses following laparotomy due to its dependent position and rich blood supply. It is essential for surgeons and physicians to be aware of this common complication and to take appropriate measures to prevent and manage it.

**Correct Answer: C. Pelvis.**