What structure is involved in forming the fetal portion of the placenta?

The villous chorion is the correct answer because it is the structure responsible for forming the fetal portion of the placenta. The villous chorion develops from the trophoblast cells of the blastocyst and forms finger-like projections called chorionic villi, which invade the uterine lining. These villi are critical for nutrient and gas exchange between the mother and the developing fetus, establishing a functional connection that supports fetal development throughout pregnancy.

The amniotic sac serves a different purpose, primarily acting as a protective environment for the fetus, offering cushioning and maintaining temperature, but it does not contribute to the formation of the placenta itself. The chorionic cavity is the space between the chorion and the amnion and plays a role in accommodating the growing fetus but is not involved in the placental structure. The decidua basalis, part of the maternal endometrium, contributes to the maternal side of the placenta but does not form the fetal portion. Thus, the villous chorion is specifically defined as the structure that develops into the fetal component of the placenta, making it the correct answer.