In human female, the blastocyst
Gets implanted in endometrium by the trophoblast cells.
Gets nutrition from uterine endometrial secretion only after implantation.
Forms placenta even before implantation.
Gets implanted into uterus 3 days after ovulation.

Implantation is initiated, when the blastocyst comes into contact with the uterine wall. The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass, which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoele. The trophoblast gives rise to the placenta. In humans, blastocyst formation begins about 5 days after fertilization.Thus, the correct option is (A).