Embro is the developmental stage of an animal when they are in the uterus or the egg. They are protected by the sac-like structure called the placenta. Thus, option D is correct.
The placenta is an organ attached to the uterus's wall and surrounds the embryo in the uterus. It provides structural support, nutrients in the form of amniotic fluid, and oxygen to the growing embryo. It is a temporary organ that gets eliminated after birth.
Therefore, the placenta protects the embryo.
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