**Core Concept**
Ovarian tumors can arise from the surface epithelium, sex cord-stromal cells, or germ cells. This diversity in origin contributes to the heterogeneity of ovarian tumors and their varying clinical presentations.

**Why the Correct Answer is Right**
The correct answer is **germ cells**. Germ cell tumors, such as dysgerminomas, yolk sac tumors, and immature teratomas, originate from the primordial germ cells that give rise to the oocytes. These tumors are more common in younger women and often present with a mass in the adnexal region. They can also be associated with elevated levels of tumor markers such as alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG).

**Why Each Wrong Option is Incorrect**
**Option A:** Surface epithelial tumors, while the most common type of ovarian cancer, account for only about 90% of ovarian tumors, not the majority of ovarian tumors.

**Option B:** Sex cord-stromal tumors, which include granulosa cell tumors and Sertoli-Leydig cell tumors, are less common than germ cell tumors.

**Option C:** Epithelial tumors, which include serous and mucinous tumors, are the most common type of ovarian cancer but do not account for the majority of ovarian tumors.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that germ cell tumors often present in younger women and can be associated with elevated tumor markers, which can aid in diagnosis.

**Correct Answer: D. Germ cells.**