Answer:
The best answer is option B. Formation of the three germinal layers and the primitive gut.
Explanation:
During early embryonic development, single-layered blastula reorganized or fold inwards to form a multilayered structure, known as gastrula. This process is known as gastrulation.
During gastrulation, three germ layers- ectoderm, mesoderm, and endoderm and internal part known as primitive gut or archenteron are formed in triploblastic animals.
Later on, ectoderm gives rise to the nervous system, epidermis, and neural crest; mesoderm gives rise to bone, muscle, and connective tissue; and the endoderm gives rise mainly to the digestive system and respiratory system.