## Core Concept
The question tests knowledge of ovarian malignancies, specifically focusing on the most common type found in young women. Ovarian cancers are classified based on the cell of origin, and the most common types include epithelial, germ cell, and sex cord-stromal tumors.

## Why the Correct Answer is Right
Germ cell tumors are the most common ovarian malignancies in young women, particularly in those under the age of 20. These tumors arise from the cells that produce eggs and include several subtypes such as dysgerminomas, yolk sac tumors, and teratomas. Among these, **dysgerminoma** is a notable example, being the most common malignant germ cell tumor of the ovary.

## Why Each Wrong Option is Incorrect
* **Option A:** Epithelial tumors are the most common type of ovarian cancer overall but are more frequently found in postmenopausal women, making them less likely to be the correct answer for young women.
* **Option B:** Sex cord-stromal tumors, which include granulosa cell tumors and Sertoli-Leydig cell tumors, are rare and not the most common type in young women.
* **Option D:** Other rare types of ovarian tumors might be considered, but they are not as prevalent as germ cell tumors in young women.

## Clinical Pearl / High-Yield Fact
A key point to remember is that ovarian germ cell tumors are highly sensitive to chemotherapy, which significantly improves the prognosis for young women diagnosed with these tumors.

## Correct Answer Line
**Correct Answer: C. Germ cell tumor.**