**Core Concept**
Epithelial ovarian cancer is the most common type of ovarian malignancy, accounting for approximately 90% of cases. This type of cancer arises from the epithelial lining of the ovary, which is derived from the coelomic epithelium.

**Why the Correct Answer is Right**
Epithelial ovarian cancer is the most common type of ovarian cancer due to its association with the epithelial lining of the ovary. The coelomic epithelium, which gives rise to the epithelial lining of the ovary, is prone to genetic mutations that can lead to cancer. The development of epithelial ovarian cancer is often linked to the accumulation of genetic mutations in the TP53 and BRCA1/2 genes, which are tumor suppressor genes that play a crucial role in maintaining genomic stability.

**Why Each Wrong Option is Incorrect**
**Option A:** Serous borderline tumors are a type of epithelial ovarian tumor, but they are not considered malignancies. They are characterized by the presence of malignant-appearing cells that lack the usual features of malignancy, such as invasion.
**Option B:** Germ cell tumors are a type of ovarian cancer, but they are relatively rare, accounting for only 2-5% of ovarian cancers. They arise from the germ cells of the ovary and can occur at any age.
**Option C:** Sex cord-stromal tumors are a type of ovarian cancer that arise from the stromal cells of the ovary. They are relatively rare and account for only 5-10% of ovarian cancers.

**Clinical Pearl / High-Yield Fact**
Epithelial ovarian cancer is often asymptomatic in its early stages, making it a challenging diagnosis to make. The presence of a palpable abdominal mass, ascites, or pelvic pain in a postmenopausal woman should raise suspicion for epithelial ovarian cancer.

**Correct Answer: C. Sex cord-stromal tumors are a type of ovarian cancer that arise from the stromal cells of the ovary. They are relatively rare and account for only 5-10% of ovarian cancers.**