Option (3) is the correct answer because both Assertion and Reason are true.

Implantation is embedding of the blastocyst into endometrium of uterus.

Correct explanation of reason is

Corpus luteum secretes large amount of progesterone which is essential for maintenance of endometrium of uterus. In absence of fertilisation, the corpus luteum degenerates hence the decrease in the level of progesterone hormone will cause disintegration of endometrium leading to menstruation.