**Core Concept**
Epithelial ovarian tumors are the most common type of ovarian cancer. They arise from the surface epithelium of the ovary and account for approximately 90% of all ovarian malignancies.

**Why the Correct Answer is Right**
The correct answer is mucinous cystadenocarcinoma, a type of epithelial ovarian tumor. Mucinous cystadenocarcinoma is the most common malignant ovarian tumor, accounting for about 40% of epithelial ovarian cancers. It is characterized by the production of mucin, a complex carbohydrate, which leads to the formation of large cysts filled with mucin. This type of tumor often presents as a large abdominal mass and can cause abdominal pain, bloating, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Serous cystadenocarcinoma is another type of epithelial ovarian tumor, but it is less common than mucinous cystadenocarcinoma.
**Option B:** Germ cell tumors, such as dysgerminoma, are less common than epithelial ovarian tumors and typically occur in younger women.
**Option C:** Sex cord-stromal tumors, such as granulosa cell tumor, are rare and typically occur in postmenopausal women.

**Clinical Pearl / High-Yield Fact**
Mucinous cystadenocarcinoma often presents as a large abdominal mass and can cause abdominal pain, bloating, and weight loss. It is essential to consider ovarian cancer in the differential diagnosis of any postmenopausal woman with an abdominal mass.

**Correct Answer: C. Mucinous cystadenocarcinoma**