**Core Concept**
The question pertains to the classification and incidence of ovarian germ cell tumors, which are a type of ovarian neoplasm that arises from the germ cells. These tumors are typically seen in younger women and can be either benign or malignant.

**Why the Correct Answer is Right**
The most common malignant germ cell tumor of the ovary is the dysgerminoma. Dysgerminomas are aggressive, high-grade tumors that arise from the primitive germ cells of the ovary. They are characterized by the presence of large, rounded cells with clear cytoplasm and a large nucleus. Dysgerminomas are highly sensitive to radiation therapy and chemotherapy, making them a curable disease if diagnosed early.

**Why Each Wrong Option is Incorrect**
**Option A:** Yolk sac tumor is a malignant germ cell tumor, but it is less common than dysgerminoma, typically accounting for 10-15% of all ovarian germ cell tumors.
**Option B:** Endodermal sinus tumor is another name for yolk sac tumor, which is a type of malignant germ cell tumor, but not the most common one.
**Option C:** Choriocarcinoma is a rare malignant germ cell tumor that is usually associated with pregnancy, and is not the most common type of ovarian germ cell tumor.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that ovarian germ cell tumors are highly sensitive to chemotherapy and radiation therapy, making early diagnosis and treatment crucial for a favorable outcome.

**Correct Answer: D. Dysgerminoma. Dysgerminoma**