Which of these is NOT a characteristic of organisms classified into Kingdom Animalia?

The correct answer is unicellular.

Organisms in Kingdom Animalia are primarily multicellular, meaning they are made up of multiple cells. While there are unicellular eukaryotic organisms (like some protists), animals are characterized by their multicellular structures. The other options—eukaryotic (having complex cells), heterotroph (relying on other organisms for food), and no cell walls—are all characteristics of animals.