**Core Concept**
The question is testing knowledge of ovarian germ cell tumors, a specific category of ovarian neoplasms. These tumors originate from the primordial germ cells and can be classified into different types based on their histological characteristics.

**Why the Correct Answer is Right**
The correct answer is **D. Dysgerminoma**. Dysgerminoma is the most common pure germ cell tumor of the ovary, accounting for approximately 40% of all ovarian germ cell tumors. It is a malignant tumor that arises from the germ cells and is often seen in young women. The pathogenesis of dysgerminoma is thought to involve mutations in the KIT gene, leading to the activation of the KIT receptor tyrosine kinase. This results in the proliferation of tumor cells and the formation of a solid tumor mass. Dysgerminoma is typically treated with surgery and may require adjuvant chemotherapy in cases of advanced disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a pure germ cell tumor. Many ovarian tumors are mixed germ cell tumors, containing both germ cell and sex cord-stromal components.
* **Option B:** This option is incorrect because it refers to a type of ovarian tumor that is not a germ cell tumor. Epithelial ovarian cancers are the most common type of ovarian cancer, but they arise from the epithelial cells that cover the surface of the ovary, not from the germ cells.
* **Option C:** This option is incorrect because it refers to a type of ovarian tumor that is not a germ cell tumor. Sex cord-stromal tumors, such as granulosa cell tumors, arise from the stromal cells of the ovary and are not germ cell tumors.

**Clinical Pearl / High-Yield Fact**
Dysgerminoma is often associated with a good prognosis, especially when diagnosed at an early stage. However, it can be challenging to distinguish from other types of ovarian tumors based on clinical presentation and imaging studies alone.

**Correct Answer: D. Dysgerminoma**