**Core Concept**
The development of endometrial cancer (ca endometrium) is influenced by a combination of genetic, hormonal, and environmental factors. The risk factors can be broadly categorized into those that increase the exposure of the endometrium to unopposed estrogen, and those that have a direct oncogenic effect.

**Why the Correct Answer is Right**
Unopposed estrogen exposure leads to hyperplasia and subsequent neoplasia of the endometrium. This can occur due to various factors such as obesity, which increases the conversion of androstenedione to estrone, and tamoxifen use, which acts as an estrogen agonist in the endometrium. On the other hand, factors like unopposed progesterone use can actually decrease the risk of endometrial cancer due to its opposing effects on estrogen.

**Why Each Wrong Option is Incorrect**
**Option A:** Obesity is a well-established risk factor for endometrial cancer due to its association with increased estrogen production and decreased insulin sensitivity.
**Option B:** Tamoxifen is an estrogen receptor modulator that acts as an agonist in the endometrium, leading to an increased risk of endometrial cancer.
**Option C:** Nulliparity (never having given birth) is a known risk factor for endometrial cancer, as it results in a longer exposure to unopposed estrogen.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the risk of endometrial cancer is increased in patients with a history of tamoxifen use, obesity, and nulliparity, and decreased in those with unopposed progesterone use.

**Correct Answer: D. Unopposed progesterone use.**