Answer choice B is Correct.

The question is asking what structure would be affected if the embryonic mesoderm were defective. This is really a matter of memorizing which structures are derived from each of the germ layers (ectoderm, mesoderm, endoderm). Choice, A, the nervous system, is derived from ectodermal precursors, as is the skin and the lens of the eye. Choice C, the stomach lining, is derived from the endodermal germ layer, which also gives rise to other linings of the digestive and respiratory tracts, as well as differentiating into the major glands of the body such as the pancreas and the liver (choice D). This leaves choice B, the heart, as the structure which is mesodermal in origin (along with the notochord, skeleton, muscle, outer coverings of internal organs, and reproductive organs).