Ans.
The amnion sac is a thin bag of water that covers developing embryo in amniotes. The main function of amniotic sac is to provide protection to the developing embryo. The amniotic fluid, present inside the sac surrounds and cushions the embryo and provides protection from physical harm. It also permits the freely movement of fetus with the uterus walls.
Thus, the correct answer is 'embryo sac.'