Which layer develops first during embryonic development?
A. Ectoderm
B. Mesoderm
C. Endoderm
D. Both A and C

Hint:- In animals, a germ layer is a primary cell layer that is formed during embryogenesis. Especially pronounced are the three germ layers in vertebrates- endoderm, mesoderm, and ectoderm.

Complete step-by-step solution:-
One of the germ layers developed during animal embryogenesis is the endoderm. The inner layer of the gastrula, which grows into the endoderm, is formed by cells migrating inwards along the archenteron. It is the first layer that needs to be created.
The correct answer is, therefore, option C.

Additional information:
As the embryo develops and contributes to the formation of all organs and tissues, a germ layer is a group of cells in an embryo that interact with each other. Two or three germ layers are produced by all species, except maybe sponges. Early in embryonic life, through the process of gastrulation, the germ layers form. A hollow cluster of cells called a blastula reorganizes into two primary layers of germs during gastrulation: an inner layer, called the endoderm, and an outer layer, called the ectoderm. There are only two main germ layers in diploblastic species; these species usually have several symmetrical body axes (radial symmetry), as is the case with jellyfish, sea anemones, and the rest of the phylum Cnidaria. All other species are triploblastic, as a third germ layer called mesoderm is formed by endoderm and ectoderm interacting. Together, from the skin and hair to the digestive tract, the three germ layers will give rise to every organ in the body.

Note:- The lining of the digestive tract, as well as that of the lungs and thyroid, will form the endoderm formed during gastrulation. For animals with three germ layers, interactions between the two germ layers cause mesoderm formation after the endoderm and ectoderm have developed.