**Core Concept**
The underlying principle being tested is the appropriate staging investigation for endometrial cancer, specifically in a post-menopausal woman with a confirmed diagnosis of moderately differentiated adenocarcinoma. **Endometrial cancer staging** is crucial for determining the extent of disease and guiding treatment.

**Why the Correct Answer is Right**
Although the specific correct answer choice is not provided, in the context of endometrial cancer, particularly with a diagnosis of moderately differentiated adenocarcinoma, **MRI (Magnetic Resonance Imaging)** is often considered the most appropriate staging investigation. This is because MRI can accurately assess the depth of myometrial invasion, cervical involvement, and lymph node status, which are critical factors in determining the stage of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific option, generally, a less sensitive imaging modality or an inappropriate investigation for staging endometrial cancer would be incorrect.
**Option B:** Similarly, an option that does not provide comprehensive information about the extent of disease would be inappropriate.
**Option D:** An option that is not relevant to staging or is less accurate than the correct choice would be incorrect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **endometrial cancer** is staged surgically, according to the FIGO (International Federation of Gynecology and Obstetrics) staging system, but preoperative imaging, particularly **MRI**, plays a crucial role in planning the surgical approach and predicting the likelihood of lymph node involvement.

**Correct Answer:** D. MRI of the pelvis.