**Core Concept**
The question is testing knowledge of ovarian germ cell tumors, specifically the most common type. Ovarian germ cell tumors arise from the germ cells, which are the cells responsible for producing eggs. These tumors can be pure or mixed and are classified into several subtypes, including dysgerminomas, yolk sac tumors, embryonal carcinomas, and teratomas.

**Why the Correct Answer is Right**
The correct answer is **D. Dysgerminoma**. Dysgerminomas are the most common pure germ cell tumor of the ovary, accounting for approximately 40% of all ovarian germ cell tumors. They are typically benign and have a good prognosis, especially when diagnosed at an early stage. Dysgerminomas arise from the primordial germ cells and are thought to be related to the male germ cell tumor, seminoma. They are characterized by the presence of large cells with clear cytoplasm and a high mitotic rate.

**Why Each Wrong Option is Incorrect**
**Option A:** Yolk sac tumor is the most common malignant germ cell tumor of the ovary, but it is not the most common pure germ cell tumor overall.
**Option B:** Embryonal carcinoma is a rare type of ovarian germ cell tumor and is often mixed with other types.
**Option C:** Teratomas are the most common type of germ cell tumor, but they are often mixed with other types, not pure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ovarian germ cell tumors are relatively rare, but they are more common in younger women, typically under the age of 30. A high index of suspicion is necessary for early diagnosis and treatment.

**Correct Answer:** D. Dysgerminoma