**Core Concept**
The question is testing the student's knowledge of ovarian germ cell tumors, specifically the most common type. Germ cell tumors arise from the cells that give rise to the eggs in the ovary, and they can be pure (consisting of a single cell type) or mixed (containing multiple cell types).

**Why the Correct Answer is Right**
Dysgerminoma is the most common pure germ cell cancer of the ovary, accounting for approximately 40% of all ovarian germ cell tumors. It is a malignant tumor that arises from the primitive germ cells of the ovary. Dysgerminomas are highly sensitive to radiation and chemotherapy, and they often present with a large tumor size and elevated tumor markers such as lactate dehydrogenase (LDH).

**Why Each Wrong Option is Incorrect**
* **Option A:** Yolk sac tumor is a type of germ cell tumor, but it is more commonly associated with the testes and is the most common type of germ cell tumor in children. It is not the most common pure germ cell cancer of the ovary.
* **Option B:** Choriocarcinoma is a rare type of germ cell tumor that is more commonly associated with the uterus and is often seen in the context of a molar pregnancy. It is not the most common pure germ cell cancer of the ovary.
* **Option C:** Embryonal carcinoma is a type of germ cell tumor that can occur in the ovary, but it is less common than dysgerminoma. It is often seen in the context of a mixed germ cell tumor.

**Clinical Pearl / High-Yield Fact**
Dysgerminoma is often associated with a large tumor size and elevated tumor markers such as LDH, and it is highly sensitive to radiation and chemotherapy.

**Correct Answer:** D. Dysgerminoma