What structure surrounds the plasma membrane of the egg in mammals?

The zona pellucida is the correct answer as it is the glycoprotein layer that surrounds the plasma membrane of an egg (oocyte) in mammals. This structure plays a crucial role in fertilization, as it is involved in the binding and recognition of sperm, preventing polyspermy (the entry of multiple sperm into one egg), and facilitating the initial stages of embryo development.

The zona pellucida's composition allows it to protect the egg and provides the necessary environment for sperm to interact with the egg during fertilization. After fertilization occurs, the zona pellucida remains in place until the embryo is ready for implantation.

In contrast, the chorion, yolk sac, and amnion are extraembryonic structures that are not directly associated with the egg itself. The chorion is involved in forming the outer membrane of the placenta, the yolk sac provides nutrients to the developing embryo in early stages, and the amnion surrounds and protects the embryo in the amniotic cavity. These functions and locations distinguish them from the zona pellucida, which is specifically associated with the egg's surface prior to fertilization and during the early stages of embryo development.