The correct option is C coitus
Emergency contraceptive must be used within 72 hours after coitus to prevent pregnancy. It prevents pregnancy by temporarily stopping ovulation. Sperms can live inside the uterus for 2-3 days, a maximum of 6 days after coitus. If ovulation is prevented during this period, fertilization and hence pregnancy can be prevented.
It is ineffective if taken after implantation has taken place.