Single celled eukaryotes are included in

Protists are unicellular eukaryotic organisms.

Single-celled eukaryotes are classified under the kingdom Protista. Protista is a diverse group that includes organisms that are primarily unicellular and have a distinct nucleus and other membrane-bound organelles, characteristic of eukaryotic cells. Examples of protists include algae, protozoans, and slime molds. This kingdom serves as a classification for organisms that don't fit into the other eukaryotic kingdoms like Plantae, Animalia, or Fungi, especially those that are unicellular or simple multicellular organisms. Hence, the correct answer is option A: Protista.