Answer: The correct answer for the blank is - B) protists.
Protist is a separate kingdom of eukaryotes, organisms of which neither belong to plants nor they belong to animals or fungi kingdom.
They are mostly unicellular, microscopic eukaryotic organisms. Some protists are multicellular but they do not show cellular differentiation that is all the cells in their body are same and are not specialized for separate function ( unlike cells of plants and animals).
Example of protist- Amoeba, Paramecium.
Thus, protists is the right answer.