The correct answer is option C, the placenta.

The placenta is an organ that develops during pregnancy and is attached to the wall of the uterus. It plays a crucial role in providing oxygen and nutrients to the developing fetus and removing waste products from the fetus's blood. It acts as a barrier and prevents the mixing of the blood of the fetus with that of the mother. Let's discuss in detail how the placenta accomplishes this important task.

The Placenta:

The placenta is a complex organ that is formed from both maternal and fetal tissues. It consists of several layers, including the maternal side (decidua) and the fetal side (chorion). The placenta is connected to the fetus by the umbilical cord, which contains blood vessels that carry oxygen, nutrients, and waste products between the fetus and the placenta.

Functions of the Placenta:

1. Nutrient and Oxygen Exchange: The placenta acts as a barrier between the mother's blood supply and the fetus's blood supply. It allows for the exchange of nutrients, such as glucose and amino acids, and oxygen from the mother's blood to the fetus's blood. This ensures that the fetus receives the necessary nutrients and oxygen for growth and development.

2. Waste Removal: The placenta also facilitates the removal of waste products, such as carbon dioxide and metabolic waste, from the fetal blood to the maternal blood. These waste products are then eliminated from the mother's body through her own excretory system, such as the lungs and kidneys.

3. Hormone Production: The placenta produces hormones, such as human chorionic gonadotropin (hCG), estrogen, and progesterone, which are essential for maintaining pregnancy. These hormones help in the development of the fetus and prepare the mother's body for childbirth.

Barrier Function of the Placenta:

The placenta acts as a barrier between the maternal and fetal circulations, preventing the direct mixing of their blood. It does this through a combination of specialized structures and cell layers that separate the two blood supplies.

1. Placental Membranes: The placenta is composed of several layers, including the chorionic villi, which are finger-like projections of the fetal placental tissue. These villi are covered by a layer of specialized cells called syncytiotrophoblasts, which help in nutrient and gas exchange. The syncytiotrophoblasts form a barrier that separates the fetal blood from the maternal blood.

2. Capillary Beds: The placenta also contains a network of capillaries that are responsible for the exchange of nutrients and waste products between the maternal and fetal circulations. These capillary beds are surrounded by layers of tissue that act as a filter, allowing only certain substances to pass through.

In conclusion, the placenta is the part within the uterus that prevents the mixing of the blood of the fetus with that of the mother. It acts as a barrier between the two blood supplies, allowing for the exchange of nutrients and oxygen while preventing the direct mixing of their blood. This ensures the proper development and growth of the fetus during pregnancy.