The correct option is B Chorion only
Once, the blastocyst is formed, the cells of the trophoblast differentiates into an inner layer and an outer layer. The outer layer is called chorion and forms the chorionic villi, which are the finger-like projections, which grow into the endometrium. An intimate connection is established between chorionic villi and uterine tissue and form a structural and functional unit between developing embryo and maternal body called placenta.
Thus, the correct answer is option (B).