Final answer:
The mesoderm is the embryonic germ layer that contributes to the development of the skeleton and muscles in a frog, distinct from the ectoderm which develops into the nervous system and skin, and the endoderm that forms the gastrointestinal and respiratory tracts.
Step-by-step explanation:
In the development of a frog, both the skeleton and muscles are involved in the developmental process which originates from specific embryonic germ layers. The correct answer to which embryonic germ layers contribute to the development of the skeleton and muscles is Mesoderm only (D).
The mesoderm is the middle germ layer in triploblastic organisms, situated between the ectoderm and endoderm. This germ layer gives rise to all specialized muscle tissues, the skeletal system, connective tissues, the circulatory system, and more. In contrast, the ectoderm gives rise to the central and peripheral nervous systems, sensory organs, and the epidermis. The endoderm primarily gives rise to the lining of the gastrointestinal tract and respiratory tract, as well as the liver and pancreas.
Therefore, in the context of frog embryonic development, the mesoderm is responsible for forming the musculoskeletal structures that underpin the frog's physical form and function.