Final answer:
The placental barriers allow the transfer of nutrients, respiratory gases, wastes, and alcohol, but not blood cell(option c).
Step-by-step explanation:
In reference to what may pass through the placental barriers, the correct option is C) nutrients, respiratory gases, wastes, and alcohol. Substances such as oxygen, carbon dioxide, and lipid-soluble substances including alcohol can pass through the placenta via simple diffusion.
Water-soluble substances like glucose utilize facilitated diffusion, while amino acids and iron are transferred through active transport. It is important to note that maternal and fetal blood do not mix; however, there is an exchange of various substances through the placental barriers. Blood cells, specifically, do not pass across the placental barriers due to the distinct separation that prevents the mother's immune cells from attacking the fetus and also stops the fetal blood cells from entering the mother's circulation.