**Core Concept**
Koener's tumor refers to a type of benign ovarian tumor, specifically a mature cystic teratoma. These tumors are characterized by their mixed composition of various tissue types, including epithelial, mesenchymal, and neural elements. The pathogenesis of Koener's tumor is thought to be related to the abnormal development of germ cells in the ovaries.

**Why the Correct Answer is Right**
Koener's tumor is a type of germ cell tumor that arises from the ovary. The tumor's composition is a result of the abnormal growth and differentiation of germ cells, leading to the formation of various tissue types within the tumor. The presence of these tissue types, such as skin, hair, muscle, and bone, is a hallmark of Koener's tumor. The tumor's benign nature is often confirmed by its encapsulated appearance and the absence of malignant features.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Koener's tumor is not typically associated with a specific age group or demographic.
**Option B:** This option is incorrect because Koener's tumor is not a type of sex cord-stromal tumor, which includes tumors such as granulosa cell tumors and Sertoli-Leydig cell tumors.
**Option C:** This option is incorrect because Koener's tumor is not a type of epithelial ovarian tumor, which includes tumors such as serous and mucinous cystadenomas.

**Clinical Pearl / High-Yield Fact**
Koener's tumor is often associated with a high risk of torsion, which can lead to ovarian ischemia and infarction. Therefore, it is essential to consider the possibility of Koener's tumor in the differential diagnosis of ovarian torsion.

**Correct Answer: D. Mature cystic teratoma**