Answer:
a.gastrulation is the correct answer.
Explanation:
The characteristics unique to animals is gastrulation
Gastrulation is a stage early in the embryonic growth in most of the animals, through which simple layered blastula is rearranged into a multi-layered structure called gastrula.
gastrulation results in the formation of three germ layers
The embryonic tissue layers are developed and arranged in their proper places during the gastrulation process.
