**Core Concept**
Tamoxifen is a selective estrogen receptor modulator (SERM) that acts as an estrogen agonist in some tissues and an antagonist in others. It is primarily used in the treatment and prevention of breast cancer. In the uterus, tamoxifen acts as an estrogen agonist, leading to unopposed estrogenic effects.

**Why the Correct Answer is Right**
Tamoxifen causes endometrial hyperplasia due to its estrogen agonist activity in the uterus. This is because tamoxifen binds to estrogen receptors in the endometrium, stimulating cell proliferation and leading to an increased risk of endometrial hyperplasia and cancer. The risk of endometrial cancer is higher in postmenopausal women taking tamoxifen, especially those with a history of estrogen-dependent tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteoporosis is not a direct effect of tamoxifen. While tamoxifen may have some adverse effects on bone density, its primary concern is endometrial hyperplasia, not osteoporosis.
**Option C:** Ovarian cancer is not a common side effect of tamoxifen. In fact, tamoxifen may have some protective effects against ovarian cancer.
**Option D:** Decreased triglyceride level is not a known effect of tamoxifen. In fact, tamoxifen may increase triglyceride levels, particularly in postmenopausal women.

**Clinical Pearl / High-Yield Fact**
Tamoxifen is often associated with an increased risk of endometrial cancer, particularly in postmenopausal women. Patients taking tamoxifen should be closely monitored for signs and symptoms of endometrial hyperplasia, such as abnormal vaginal bleeding.

**✓ Correct Answer: B. Endometrial hyperplasia**