**Core Concept**
Granulosa cell tumors are a type of sex cord-stromal tumor that originates from the granulosa cells of the ovary. They are characterized by their ability to produce estrogen, leading to clinical manifestations such as endometrial hyperplasia and bleeding.

**Why the Correct Answer is Right**
The correct marker for granulosa cell tumors is inhibin, which is a protein produced by the granulosa cells. Inhibin is composed of alpha and beta subunits, and it plays a crucial role in regulating the production of follicle-stimulating hormone (FSH) by inhibiting its secretion. The alpha subunit of inhibin is also known as inhibin-alpha, which is the marker used for diagnosing granulosa cell tumors. This is because the tumor cells produce high levels of inhibin-alpha, which can be detected in the serum or tumor tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** CA-125 is a tumor marker for epithelial ovarian cancers, not granulosa cell tumors.
**Option B:** Beta-hCG is a marker for choriocarcinoma and other trophoblastic neoplasms, not granulosa cell tumors.
**Option C:** AFP (alpha-fetoprotein) is a marker for yolk sac tumors and hepatocellular carcinoma, not granulosa cell tumors.

**Clinical Pearl / High-Yield Fact**
Inhibin-alpha is a specific marker for granulosa cell tumors, and its detection can help differentiate these tumors from other types of ovarian neoplasms. However, it's essential to note that inhibin-alpha levels can also be elevated in other conditions, such as pregnancy and polycystic ovary syndrome (PCOS).

**Correct Answer: D. Inhibin-alpha.**