Q. In mammals, one of the following forms the major part of the placenta

Explanation by: Official MCQ Buddy

In mammals, the chorion forms the major part of the placenta, which facilitates the exchange of nutrients, gases, and waste between the mother and the fetus. The placenta is a vital organ for fetal development.
Why Other Options Are Incorrect:
(A) Yolk-sac → Incorrect
In mammals, the yolk sac is small and provides early nutrition but does not form the placenta.
(B) Amnion → Incorrect
The amnion forms a protective fluid-filled sac around the embryo but is not involved in placental formation.
(D) Allantois → Incorrect
The allantois helps in waste storage and later contributes to the umbilical cord but does not form the placenta.
Thus, the chorion is the primary structure involved in placental formation in mammals.