Final answer:
The liver (option D) is not formed by the mesoderm, as it originates from the endoderm.
Step-by-step explanation:
The question revolves around understanding which structures are not formed by one of the three primary germ layers during embryonic development, specifically the mesoderm. The mesoderm is responsible for giving rise to various structures in the embryo, including the cardiac muscle, kidneys, and the dermis of the skin. Based on the information provided, the liver is not formed by the mesoderm but instead, it is derived from the endoderm, which also gives rise to the epithelial lining of the gastrointestinal tract and the respiratory tract. Therefore, the correct answer to the student's question is (d) liver, as it is not formed by the mesoderm.