Question

the clump of 16 or more cells, about 72 hours after fertilization...
zygote
morula
blastula
gastrula

Explanation:

In embryology (a subfield of Biology, which is part of Natural Science), a morula is a solid clump of 16 or more cells that forms about 72 hours after fertilization. A zygote is a single - celled fertilized egg, a blastula is a hollow ball of cells that forms after the morula, and a gastrula is formed from the blastula during further embryonic development with distinct germ layers. So the correct answer is Morula.

Answer:

B. Morula