What is the one characteristic used to place organisms in kingdoms is?

The one characteristic used to place organisms in kingdoms is their cellular organization, specifically whether they are prokaryotic or eukaryotic. Prokaryotes, which lack a nucleus and membrane-bound organelles, are classified into the domains Bacteria and Archaea, while eukaryotes, which have a defined nucleus, are categorized into kingdoms such as Animalia, Plantae, Fungi, and Protista. This fundamental distinction helps in organizing the vast diversity of life on Earth.