**Core Concept:** Ovarian tumours are classified into germ cell tumours, stromal tumours, and epithelial tumours based on their cell of origin. Germ cell tumours are primarily feminizing due to the production of sex steroids. Stromal tumours can also produce sex steroids and contribute to feminization, while epithelial tumours primarily cause masculinization.

**Why the Correct Answer is Right:** The correct answer, option D (Germ cell tumours), is right because germ cell tumours primarily consist of cells derived from primordial germ cells and are the primary cause of feminizing ovarian tumours. These tumours secrete sex steroids like estradiol, which is primarily responsible for feminizing effects.

**Why Each Wrong Option is Incorrect:**

A. Stromal tumours (Option B) can also produce sex steroids, but they are not exclusively feminizing. While they can cause feminization, they are less likely to cause significant feminization compared to germ cell tumours.

B. Epithelial tumours (Option C) primarily cause masculinization due to the production of androgens, not feminization.

C. None of the options (Option A) are entirely incorrect as germ cell tumours are the primary cause of feminizing ovarian tumours.

**Clinical Pearl / High-Yield Fact:** In clinical practice, distinguishing between germ cell tumours, stromal tumours, and epithelial tumours is crucial as treatment strategies differ significantly based on the type of tumour. Germ cell tumours typically require chemotherapy, while stromal and epithelial tumours often require surgery and/or hormone therapy.