Is a flatworm multicellular?

The flatworm is one portion of the phylum Platyhelminthes. "Platy" meaning flat, and "helminthes" translating to worm. All species belonging to the Platyhelminthes phylum have two cell layers because...

flatworms, a phylum of relatively simple soft-bodied invertebrate animals

multicellular means to have more than one cell. For example, we are multicellular, but on the other hand if something only has one cell, then it is considered unicellular

The differentiation of the cells. Cells in plants are doing many types of jobs, while cells in protists are basically all the same.

Yes