**Core Concept**
Germ cell tumors are a type of ovarian tumor that originates from the cells that produce eggs in the ovaries. These tumors are more common in younger women, particularly in those under the age of 20. They can be benign or malignant and may produce hormones, leading to various symptoms.

**Why the Correct Answer is Right**
Germ cell tumors are the most common type of ovarian tumor in women under 20 years of age. This is because these tumors arise from the cells that produce eggs, which are present in higher numbers during childhood and adolescence. The most common types of germ cell tumors in this age group include dysgerminomas, yolk sac tumors, and immature teratomas. These tumors can grow rapidly and may produce symptoms such as abdominal pain, bloating, and changes in menstrual patterns.

**Why Each Wrong Option is Incorrect**
**Option A:** Epithelial tumors are the most common type of ovarian tumor overall, but they are more common in older women, typically over the age of 50. They arise from the surface cells of the ovary and are often associated with genetic mutations.
**Option C:** Metastatic tumors are not a primary type of ovarian tumor, but rather a secondary tumor that has spread to the ovary from another site in the body. While metastatic tumors can occur in the ovary, they are not the most common type in women under 20.
**Option D:** Sex cord stromal tumors are a rare type of ovarian tumor that arises from the connective tissue of the ovary. They can produce hormones and may cause symptoms such as hirsutism and virilization, but they are not the most common type in women under 20.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that germ cell tumors are more common in younger women and may produce symptoms such as abdominal pain and changes in menstrual patterns. Dysgerminomas, yolk sac tumors, and immature teratomas are the most common types of germ cell tumors in this age group.

**✓ Correct Answer: B. Germ cell tumour**