The correct answer is Option: B) Implantation
Implantation is defined as the attachment of fertilized egg or blastocysts to the wall of uterus at the beginning of pregnancy. Occurrence of implantation ensures the women to be described as pregnant.
Other options are wrong.. e.g. Intercourse is only an act between a man and a woman and does not always result in a pregnancy. On the other hand menstruation has nothing to do witha women getting pregnant.