**Core Concept**
The question is testing the student's knowledge of ovarian germ cell tumors, specifically the most common type. Ovarian germ cell tumors arise from the cells that give rise to the eggs (oocytes) and can be pure or mixed types.

**Why the Correct Answer is Right**
The most common pure germ cell tumor of the ovary is the dysgerminoma. Dysgerminomas are highly malignant tumors that arise from the germ cells of the ovary. They are typically composed of large, uniform, round cells and are often associated with elevated levels of lactate dehydrogenase (LDH). Dysgerminomas are sensitive to radiation and chemotherapy and are usually treated with surgery, radiation, or a combination of both.

**Why Each Wrong Option is Incorrect**
**Option A:** Yolk sac tumor is a type of germ cell tumor, but it is not the most common pure germ cell tumor of the ovary. Yolk sac tumors are highly malignant and are often associated with elevated levels of alpha-fetoprotein (AFP).
**Option B:** Teratoma is a type of germ cell tumor, but it is not the most common pure germ cell tumor of the ovary. Teratomas are composed of multiple tissue types and can be benign or malignant.
**Option C:** Choriocarcinoma is a rare type of germ cell tumor that is often associated with elevated levels of human chorionic gonadotropin (hCG). It is not the most common pure germ cell tumor of the ovary.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that dysgerminomas are highly malignant and are often associated with elevated levels of LDH. This can be a useful marker for diagnosis and monitoring of the disease.

**Correct Answer:** D. Dysgerminoma