You are answering a USMLE-style medical question. Question: A 31-year-old woman comes to the physician because of dragging pelvic pain and a 3 kg (6 lb 9 oz) weight loss over the past 6 months. Menses occur at irregular 30- to 45-day intervals; her last menstrual period was 5 weeks ago. Her temperature is 38°C (100.4°F), heart rate is 102/min, and blood pressure is 128/84 mm Hg. Physical examination shows hyperreflexia. Urine pregnancy test is negative. Ultrasonography shows a 6-cm hypoechoic adnexal mass. This adnexal mass is most likely derived from which of the following cell types? Options: A) Germ cells B) Chorionic epithelial cells C) Endometrial cells D) Stromal cells Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.