**Core Concept**
The most common ovarian tumor in young individuals, particularly those under 20 years of age, is related to **germ cell tumors**, which arise from the cells that produce eggs. These tumors can be benign or malignant. In the context of ovarian tumors in this age group, **germ cell tumors** are significant due to their frequency and potential impact.

**Why the Correct Answer is Right**
Given the options provided, the correct answer is related to the type of tumor that is most prevalent among young females. **Germ cell tumors**, such as dysgerminomas, are indeed the most common type of ovarian tumor in this age group. They are often associated with a good prognosis, especially when diagnosed and treated early.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not represent the most common type of ovarian tumor in young females.
**Option B:** Similarly, this option is not accurate for the specified age group.
**Option D:** This choice is also incorrect as it does not align with the common ovarian tumors found in females under 20.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **germ cell tumors** are the most common ovarian tumors in young females, particularly those under 20. Early diagnosis and appropriate management are key to a favorable outcome.

**Correct Answer:** D. Germ cell tumor