What organism kingdom is multicellula?

Answer 1

The multicelular organisms are found in all of life's kingdoms.

Multicellular life has evolved independently dozens of times we belive, once for plants, once for animals, once for brown algae and perhaps several times for fungi, slime molds, and red algae.

Multicellular bodies exists in both prokaryotes and eukaryotes, and first appeared several billion years ago in cyanobacteria.

Thus the whole basis of your question is wrong, multicellularity is NOT a sound discrimination basis for classifying Earth's life forms.