The correct choice is (a) hymen
Easiest explanation: The hymen, the thin membranous diaphragm at the vaginal orifice, is often torn during the first coitus (intercourse). Hymen maybe torn in other ways too, like sports, a disease, sudden jolt, etc. In some females, the hymen persists even after intercourse. Hence, presence or absence of hymen is an unreliable indicator.