**Core Concept**
A postmenopausal woman presenting with bleeding per vaginum is a classic symptom complex that warrants a thorough evaluation for potential causes, including malignancy, endometrial hyperplasia, and atrophy. In this scenario, the patient's diabetes mellitus and postmenopausal status increase her risk for endometrial cancer. The endometrium is the lining of the uterus, and its hyperplasia or cancer can cause abnormal vaginal bleeding.

**Why the Correct Answer is Right**
The patient's diabetes mellitus increases her risk for endometrial cancer due to insulin resistance and hyperinsulinemia, which can stimulate the growth of endometrial cells. Additionally, the patient's postmenopausal status means she has a higher risk of endometrial atrophy, which can also cause vaginal bleeding. The endometrial cancer cells can produce estrogen-independent growth factors, leading to unopposed estrogen stimulation of the endometrium, resulting in hyperplasia and eventually cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Atrophic vaginitis is a possible cause of postmenopausal bleeding, but it is less likely given the patient's diabetes mellitus and the presence of bleeding, which is more suggestive of malignancy or hyperplasia.
**Option B:** Ovarian cancer can cause postmenopausal bleeding, but it is less common than endometrial cancer and typically presents with more systemic symptoms such as weight loss and abdominal pain.
**Option C:** Cervical cancer can also cause postmenopausal bleeding, but it is less common than endometrial cancer and typically presents with more localized symptoms such as pelvic pain and abnormal Pap smears.

**Clinical Pearl / High-Yield Fact**
In postmenopausal women, a new onset of vaginal bleeding is a "red flag" symptom that warrants further investigation, including endometrial biopsy and pelvic imaging, to rule out malignancy and hyperplasia.

**Correct Answer: D. Endometrial cancer.