## **Core Concept**
The question focuses on identifying the most common type of ovarian tumor in individuals under 20 years of age. Ovarian tumors in young individuals are predominantly germ cell tumors, which arise from the cells that produce eggs.

## **Why the Correct Answer is Right**
Germ cell tumors are the most common ovarian tumors in children and adolescents, accounting for approximately 80-90% of ovarian tumors in this age group. These tumors originate from the germ cells of the ovary and include several subtypes such as dysgerminomas, yolk sac tumors, and teratomas. **Teratomas**, specifically mature cystic teratomas (also known as dermoid cysts), are particularly common and can occur in young individuals.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While epithelial tumors are common in adults, they are less common in individuals under 20 years of age.
- **Option B:** Sex cord-stromal tumors, such as granulosa cell tumors, are rare and not the most common type in young individuals.
- **Option D:** Although other types of germ cell tumors like dysgerminomas and yolk sac tumors do occur in this age group, they are not as common as teratomas.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **mature teratomas (dermoid cysts)** are the most common ovarian tumor in young women and can sometimes cause ovarian torsion. They are typically benign but can have malignant potential, especially in older adults.

## **Correct Answer:** . Teratoma.