Final answer:
In the lacunae of the placenta, cells of the trophoblast are in direct contact with maternal blood, invading the endometrium, making the correct answer a) Trophoblast; endometrium.
Step-by-step explanation:
The question is concerning the placental structure and the interaction between maternal and embryonic tissues. In the lacunae of the placenta, which are spaces filled with maternal blood, cells of the trophoblast are in direct contact, and this tissue invades the endometrium. Therefore, the correct answer to the question is a) Trophoblast; endometrium. The trophoblast is composed of cells that form the outer layer during the early stages of embryo development and eventually develops into the fetal portion of the placenta. The placenta functions to facilitate the exchange of nutrients and wastes between the maternal and fetal bloodstreams without direct blood mixing. The chorionic villi, which are projections of the chorionic membrane that invades the endometrium, develop into the fetal part of the placenta and allow this vital exchange.