The correct option is A Umbilical cord
The placenta is connected to the embryo by the umbilical cord . The umbilical cord is a long tube-like structure, begins to form during the 5th week after conception, connecting the developing foetus to the placenta. Nicknamed as the ‘supply line,’ it transports nutrients and oxygen to the foetus and carries out the removal of foetal waste products. The blood within the umbilical cord is rich with undifferentiated stem cells.
Amnion is not correct because it is the innermost membrane that encloses the embryo.
The yolk sac is the first element seen within the gestational sac during pregnancy.
Chorion is the outermost foetal membrane around the embryo which develops from an outer fold on the surface of the yolk sac, lying outside the zona pellucida.