The correct answer is: A. by the union of foetal and uterine tissue.
The placenta is an organ that develops in the uterus during pregnancy. It is attached to the wall of the uterus by the umbilical cord and to the fetus by the umbilical arteries and vein. The placenta provides oxygen and nutrients to the fetus and removes waste products. It is also responsible for producing hormones that help to maintain the pregnancy.
The placenta is formed from the union of foetal and uterine tissue. The foetal part of the placenta is formed from the chorionic villi, which are finger-like projections that grow from the placenta into the maternal blood supply. The uterine part of the placenta is formed from the decidua basalis, which is the lining of the uterus that thickens during pregnancy.
The placenta is a temporary organ that is shed after the birth of the baby. It is usually expelled within 30 minutes to an hour after delivery.