Final answer:
The mesoderm is the embryonic layer found only in bilaterally symmetrical eumetazoans, situated between the ectoderm and endoderm.
Step-by-step explanation:
The embryonic layer found only in bilaterally symmetrical eumetazoans is the: c) Mesoderm. During embryogenesis, diploblasts develop two embryonic germ layers - an ectoderm and an endoderm. In contrast, triploblasts develop a third germ layer - the mesoderm - which arises from mesendoderm and resides between the ectoderm and endoderm. This mesoderm gives rise to many structures including the muscles, connective tissues, and the circulatory system. For example, question 23 states that the germ layer which gives rise to the heart is the mesoderm, reinforcing the variety of tissues it is responsible for.
Refuting a false statement, the mesoderm does not give rise to the central nervous system; instead, it is the ectoderm that gives rise to the central nervous system as indicated in a subsequent question. Thus, option 'd' in the set of statements about diploblasts and triploblasts is false.