**Core Concept**
Endometrial carcinoma is a type of cancer that originates in the endometrium, the inner lining of the uterus. The risk of developing endometrial carcinoma is influenced by various factors, including hormonal, genetic, and environmental factors.

**Why the Correct Answer is Right**
The correct answer is linked to the increased risk of endometrial carcinoma in women with unopposed estrogen exposure. Unopposed estrogen stimulates the proliferation of endometrial cells, leading to an increased risk of malignant transformation. This is particularly relevant in postmenopausal women who have a higher risk due to the increased levels of estrogen in the absence of progesterone. The enzyme aromatase plays a crucial role in converting androgens to estrogens, contributing to the increased estrogen levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not directly relate to the risk factors for endometrial carcinoma. While obesity is a risk factor for endometrial carcinoma, it is not the most direct association compared to unopposed estrogen exposure.
**Option B:** This option is incorrect as it is not a well-established risk factor for endometrial carcinoma. Although tamoxifen is a selective estrogen receptor modulator (SERM) that can have estrogenic effects on the endometrium, it is not a primary risk factor for endometrial carcinoma.
**Option C:** This option is incorrect as it is not directly related to the risk of endometrial carcinoma. While pelvic radiation therapy can increase the risk of endometrial carcinoma, it is not as strongly associated with the risk as unopposed estrogen exposure.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that unopposed estrogen exposure is a significant risk factor for endometrial carcinoma, particularly in postmenopausal women. This is why hormone replacement therapy (HRT) often includes a combination of estrogen and progesterone to mitigate this risk.

**Correct Answer: D. Unopposed estrogen exposure.**