**Core Concept**
Epithelial ovarian cancer is the most common type of ovarian malignancy, accounting for approximately 90% of all ovarian cancers. It arises from the surface epithelium of the ovary.

**Why the Correct Answer is Right**
Epithelial ovarian cancer is thought to originate from the transformation of ovarian surface epithelial cells, which are the cells lining the surface of the ovary. This type of cancer is often associated with genetic mutations, particularly in the BRCA1 and BRCA2 genes, which are tumor suppressor genes. The most common histological subtypes of epithelial ovarian cancer are serous, mucinous, clear cell, and endometrioid carcinomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Germ cell tumors are a less common type of ovarian malignancy, accounting for only 2-5% of all ovarian cancers. They arise from the germ cells of the ovary.
**Option B:** Sex cord-stromal tumors are a rare type of ovarian malignancy, accounting for less than 5% of all ovarian cancers. They arise from the stromal cells of the ovary.
**Option C:** Metastatic tumors to the ovary are a type of ovarian malignancy, but they are not a primary type of ovarian cancer. They occur when cancer cells from another part of the body spread to the ovary.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ovarian cancer is often asymptomatic in its early stages, making screening and early detection challenging. The American College of Obstetricians and Gynecologists (ACOG) recommends annual screening with a pelvic exam and transvaginal ultrasound for women with a high risk of ovarian cancer, such as those with a family history of the disease.

**Correct Answer:** C.