Remnant-matching angle. The trick is to know which adult female structures are leftovers of the mesonephric (Wolffian) system. The cranial part lingers as the epoophoron and paroophoron, and the caudal part lingers as the Gartner duct, whose cystic swelling in the anterolateral vaginal wall is the Gartner cyst. So the only mesonephric remnant in the list is option C. Reject the others on simple grounds: the pouch of Douglas and the uterovesical pouch are not embryological remnants at all but peritoneal recesses formed by reflections of peritoneum, and the broad ligament is a double fold of peritoneum draping the uterus and tubes. None has anything to do with the Wolffian duct. Therefore Gartner cyst is the answer. Reference: Gartner's cyst of vagina, J Anesth Crit Care Open Access, 2017.