Diploblastic refers to where .
a. Fish and amphibians, cells divide in twos synchronously
b. Sea stars and humans, development can proceed in two directions
c. Birds and reptiles, the egg must be completely nourished within a shell
d. Cnidarians and comb jellies, only two germ layers are formed
e. Only mammals, advanced neural structures are formed

d. Cnidarians and comb jellies, only two germ layers are formed

Explanation:

Diploplastic animals are the animals which do not have three germs layer so rather than three they are made up of only two germ layer which is ectoderm and endoderm. Mesoderm is absent in them so they do not have true coelom or body cavity.

So as cnidarians and comb jellies are formed from only two germ layers that are endoderm and endoderm, therefore, they are called diploblastic. Fish, amphibians, reptiles, and mammals are triploblastic animals. So the correct answer is d.