**Core Concept**
Granulosa cell tumors are a type of sex cord-stromal tumor that originate from the granulosa cells of the ovary. These cells play a crucial role in the regulation of ovarian follicular growth and steroidogenesis. The secretion of specific hormones by granulosa cell tumors is a key characteristic of these neoplasms.

**Why the Correct Answer is Right**
Granulosa cell tumors are known to secrete estrogen, which is primarily responsible for the symptoms and complications associated with these tumors. The excess estrogen produced by granulosa cell tumors can lead to endometrial hyperplasia, uterine bleeding, and other estrogen-related symptoms. The estrogen secretion is mediated by the aromatase enzyme, which converts androgens to estrogens in the tumor cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because granulosa cell tumors do not typically secrete human chorionic gonadotropin (hCG), which is more commonly associated with choriocarcinoma and other germ cell tumors.
**Option B:** This option is incorrect because alpha-fetoprotein (AFP) is a tumor marker for yolk sac tumors and hepatocellular carcinoma, not granulosa cell tumors.
**Option C:** This option is incorrect because inhibin is a protein produced by granulosa cells, but it is not the primary hormone secreted by granulosa cell tumors.

**Clinical Pearl / High-Yield Fact**
Granulosa cell tumors are often associated with a characteristic "starry sky" appearance on histopathology due to the presence of numerous lymphocytes and histiocytes scattered throughout the tumor.

**Correct Answer: D. Estrogen.**