Which organ serves as a source of nourishment for a fetus during pregnancy?

The placenta is the correct choice because it plays a crucial role in fetal development by acting as the interface between the mother and the developing fetus. It facilitates the exchange of nutrients, gases, and waste products. Through the placenta, vital substances like oxygen and glucose are delivered to the fetus, while carbon dioxide and other waste products are removed. Additionally, the placenta produces hormones that are essential for maintaining pregnancy and supporting fetal growth.

Other choices do have roles in pregnancy but do not directly provide nourishment in the way the placenta does. The uterus is primarily the organ that houses and protects the developing fetus, and while it supports the pregnancy overall, it does not function as the nutrient source. The ovary produces the eggs necessary for fertilization but is not involved in fetal nourishment during pregnancy. The amniotic sac provides a protective fluid environment for the fetus but does not provide nutrients. Thus, the placenta's unique role makes it the key organ for delivering nourishment during pregnancy.