**Core Concept**
The patient's presentation of post-menopausal bleeding, along with her history of diabetes and obesity, necessitates a careful evaluation of potential causes, including malignancy, endometrial hyperplasia, and atrophy. The patient's nulliparous status and post-menopausal bleeding increase her risk for endometrial cancer.

**Why the Correct Answer is Right**
The most likely diagnosis in this patient is endometrial cancer. The risk factors for endometrial cancer include obesity, diabetes, and nulliparity, which are all present in this patient. The presence of post-menopausal bleeding is a red flag symptom for malignancy. Endometrial cancer arises from the endometrium, the lining of the uterus, and is often associated with unopposed estrogen stimulation, which can lead to endometrial hyperplasia and eventually cancer.

**Why Each Wrong Option is Incorrect**
* **Option A:** While endometrial atrophy can cause post-menopausal bleeding, it is less likely in a patient with a history of obesity and diabetes, as these factors increase the risk for endometrial cancer.
* **Option B:** Endometrial polyps are a possible cause of post-menopausal bleeding, but they are less likely in a patient with a history of obesity and diabetes.
* **Option C:** Endometrial hyperplasia without atypia can cause post-menopausal bleeding, but it is less likely in a patient with a history of obesity and diabetes, as these factors increase the risk for endometrial cancer.

**Clinical Pearl / High-Yield Fact**
Post-menopausal bleeding is a red flag symptom for malignancy, and patients with risk factors such as obesity, diabetes, and nulliparity should undergo further evaluation, including endometrial biopsy and imaging studies.

**Correct Answer: C. Endometrial cancer.**