**Core Concept:** Ovarian tumours are benign or malignant neoplasms that develop from various cell types within the ovary. They can be classified into epithelial, germ cell, and stromal cell types based on their origin.

**Why the Correct Answer is Right:** Ovarian tumours commonly arise from germ cell cells, which are the primary component of the ovary. Germ cell tumours make up about 30-40% of all ovarian malignancies. They are derived from the undifferentiated germ cells and can develop into various types of malignant neoplasms, such as dysgerminomas, seminomas, and non-seminomatous germ cell tumours (NSGCTs).

**Why Each Wrong Option is Incorrect:**

A. Epithelial tumours are less common but include serous, mucinous, endometrioid, and clear cell carcinomas. These types of tumours do not account for the majority of ovarian malignancies.

B. Stromal cell tumours are rare and include granulosa cell tumours and theca cell tumours. They are also not the primary cause of ovarian tumours.

C. Ovarian teratomas are benign tumours made up of differentiated cells from three germ cell layers. These tumours are not malignant and do not represent the most common cause of ovarian tumours.

D. None of the above options accurately represents the primary cause of ovarian tumours. However, explaining each wrong option helps to solidify the correct answer.

**Clinical Pearl:** Understanding the origin of ovarian tumours is crucial for appropriate diagnosis and treatment planning. Knowing the histological classification of ovarian tumours allows physicians to determine the prognosis, treatment options, and monitor patients for disease progression.