Firstly, it cannot be bacteria because bacteria is prokaryotic with no membrane bound organelles. And it's cell wall is made of murein (peptidoglycan).
It certainly isn't a virus virus because a virus has no true cellular structure.
Let's see why we ruled out the other 3 eukaryotic kingdoms:
It can't be an animal because animal cells don't have cell walls and are multicellular.
It can't be fungi because its cell wall is made of chitin, not cellulose.
It can't be a plant because plants are all multicellular.
