Answer:
The correct answer is - the placenta, the site of exchange of materials between the mother and the fetus.
Step-by-step explanation:
In the given diagram there is a representation of a human embryo developing in the uterus. Different parts are numbered and the number three represents the site of exchange of materials between the mother and the fetus and this organ is called the placenta. It is the organ that develops between fetus and uterus during pregnancy that allows substances such as nutrients, to diffuse from the mother's blood to the fetus and connected by the umbilical cord.