**Core Concept**
The question tests the knowledge of drugs used in the management of uterine fibroids, specifically those that reduce fibroid size. Uterine fibroids are benign tumors of the uterus that can be treated with various medications, including hormone therapies and GnRH analogs.

**Why the Correct Answer is Right**
GnRH analogs work by initially stimulating the pituitary gland to release gonadotropins, which in turn stimulate the ovaries to produce estrogen and progesterone. However, prolonged use of GnRH analogs leads to downregulation of GnRH receptors in the pituitary gland, resulting in a decrease in gonadotropin release and subsequently a decrease in estrogen and progesterone production. This decrease in estrogen and progesterone levels causes a decrease in fibroid size due to the reduction in estrogen-stimulated growth of the fibroids.

**Why Each Wrong Option is Incorrect**
**Option A:** Danazol is a synthetic androgen that can be used to treat endometriosis and fibrocystic breast disease, but it is not typically used to reduce the size of fibroids. Danazol works by suppressing gonadotropin release, but it does not have a significant effect on reducing fibroid size.

**Option B:** RU-486, also known as mifepristone, is a progesterone receptor antagonist that is used for medical abortion and as an emergency contraceptive. While it can affect progesterone levels, it is not typically used to reduce the size of fibroids.

**Option C:** Estrogen is a hormone that can stimulate the growth of fibroids, so it would not be used to reduce their size. Estrogen therapy is often used to treat symptoms of menopause and postmenopausal women.

**Option D:** Progesterone is another hormone that can stimulate the growth of fibroids, so it would not be used to reduce their size. Progesterone therapy is often used to treat menstrual irregularities and infertility.

**Clinical Pearl / High-Yield Fact**
GnRH analogs can be used to reduce the size of fibroids and can be an effective treatment option for women with symptomatic fibroids. However, they can cause menopausal symptoms due to the decrease in estrogen levels, so they are typically used for a short period of time.

**Correct Answer:** C. GnRH analogs can reduce the size of fibroids by decreasing estrogen and progesterone levels, which in turn reduces the growth of the fibroids.