This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Luteal phase is best diagnosed by : \nSelect and explain. \nA. Serum progesterone levels, B. Endometrial biopsy, C. Basal body temperature, D. Ultrasonography', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Endometrial biopsy', 'role': 'assistant'}]