Among the characteristics unique to animals is

a. gastrulation c. sexual reproduction

b. multicellularity. d. flagellated sperm

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

endoderm mesoderm ectoderm

The embryonic tissue layers are developed and arranged in their proper places during the gastrulation process.