What type of microorganisms are the simplest organisms in the animal kingdom?

Protozoa are indeed recognized as some of the simplest organisms in the animal kingdom. They belong to the kingdom Protista and are single-celled eukaryotic organisms. Protozoa exhibit characteristics similar to those of animals, including motility and the ability to consume organic materials. This makes them more complex than bacteria and viruses, which are either prokaryotic or acellular, respectively.

Fungi, while also eukaryotic, are distinctly different as they belong to their own kingdom and possess characteristics, such as a chitinous cell wall, that set them apart from protozoa. Bacteria, being prokaryotes, do not fit within the animal kingdom and are fundamentally different in structure and function compared to protozoa. Viruses, on the other hand, are not classified as living organisms and require a host to replicate, which further distances them from being considered part of the animal kingdom. Protozoa stand out due to their simplicity and classification as unicellular organisms within the broader context of animals.