The correct answer is A. Oral contraceptives can reduce the risk of ovarian cancer while taking the medication. This is a well-established fact in medical literature and has been confirmed in multiple studies. Option B is incorrect because oral contraceptives actually decrease the risk of endometrial cancer. Option C is also incorrect because there is no evidence to suggest that oral contraceptives increase the risk of cervical cancer. Option D is also incorrect as there is no evidence to suggest that oral contraceptives cause significant risk of breast cancer.