**Core Concept**
Granulosa cell tumors of the ovary are a type of sex cord-stromal tumor that produce estrogen, leading to various clinical manifestations such as endometrial hyperplasia and irregular vaginal bleeding. The management and follow-up of these tumors require monitoring of estrogen levels.

**Why the Correct Answer is Right**
Calcitonin gene-related peptide (CGRP) is not the primary marker for granulosa cell tumors. However, the tumor marker most useful for follow-up of patients with granulosa cell tumors is inhibin-B. Inhibin-B is a protein produced by granulosa cells and is often elevated in granulosa cell tumors. It is a useful marker for monitoring disease recurrence or progression. Inhibin-B levels can help in distinguishing granulosa cell tumors from other ovarian tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** - This option is incorrect as alpha-fetoprotein (AFP) is a tumor marker for yolk sac tumors and hepatocellular carcinoma, not granulosa cell tumors.
**Option B:** - This option is incorrect as human chorionic gonadotropin (hCG) is a tumor marker for choriocarcinoma and germ cell tumors, not granulosa cell tumors.
**Option C:** - This option is incorrect as CA-125 is a non-specific tumor marker that can be elevated in various types of ovarian tumors, but it is not specific for granulosa cell tumors.

**Clinical Pearl / High-Yield Fact**
Inhibin-B is a useful marker for monitoring granulosa cell tumors, but its levels can be influenced by other factors such as pregnancy and menstruation. It is essential to interpret inhibin-B levels in the context of clinical presentation and imaging studies.

**Correct Answer:** C. Inhibin-B.