The correct answer is option 'A' - Placenta.

Excretion is the process by which waste products are removed from the body. In mammals, including humans, excretion is primarily carried out by the kidneys. However, in the case of a mammalian embryo, which is still developing, it does not have functional kidneys. Therefore, the excretory products of a mammalian embryo are eliminated through a different mechanism.

During embryonic development in mammals, the embryo is surrounded by protective membranes called the amnion and the chorion. These membranes are filled with a fluid called the amniotic fluid, which serves various functions including cushioning and protecting the developing embryo. However, the amniotic fluid also acts as a medium for the elimination of waste products produced by the embryo.

The waste products produced by the mammalian embryo include urea and other nitrogenous wastes. These waste products are transported from the developing embryo to the placenta through the umbilical cord. The placenta is an organ that develops during pregnancy and acts as a connection between the mother and the developing embryo. It allows for the exchange of nutrients, gases, and waste products between the mother's blood and the embryo's blood.

Once the waste products reach the placenta, they are transferred into the mother's blood through a process called diffusion. The waste products then travel through the mother's circulatory system and eventually reach her kidneys, where they are excreted from her body.

In summary, the excretory products of a mammalian embryo are eliminated through the placenta. The waste products produced by the embryo are transported to the placenta through the umbilical cord and then transferred into the mother's blood for elimination through her kidneys.