(A) are the most primitive form of eukaryotes
(B) include the greatest variety of eukaryotic organisms
(C) only unicellular
(D) only heterotrophs
(A) and (B) are correct. Because protists are the first kingdom to evolve within Domain Eukarya, it makes sense that they are the most primitive. The also include the greatest variety of eukaryotic organisms because they later involved in animals, plants, and fungi. They also just encompass any organisms that do not fit properly in any of the other eukaryotic kingdoms. They can be both unicellular and multicellular (C), heterotrophs and autotrophs (D).