Answer;
It is any organism that is not animal, plant, or fungus
Explanation;
The kingdom Protista is considered a catch all kingdom because it is any organism that is not animal, plant, or fungus. In other words it is a group for eukaryotic organisms that do not seem to fit into any other kingdoms of eukaryotic organisms.
They are said to be the most diverse of all eukaryotic kingdoms because the organisms in the Protista kingdom are eukaryotic organisms that are not plants, animals, or fungi.