In which phylum do we first see tissue-level organization?

The correct answer is- phylum- Cnidaria.

All living organisms are composed of building blocks termed as cells. Cells that perform similar functions are grouped together to form tissue.

The phylum where tissue-level of organization is seen first is Cnidaria. This phylum contains approximately 10,000 species ( such as jellyfish, sea anemones, corals).

Cnidarians have cells that are organized into two types of tissues, which occur in two layers. External tissue layer is ectoderm and inner tissue layer is endoderm.