Which of the following is NOT a domain in the three-domain system?

• A. Bacteria
• B. Archaea
• C. Protista
• D. Eukarya

Answer: (C)

Classification into domains is based on fundamental genetic differences, especially in ribosomal RNA genes, which reveal three broad lineages of life. These three domains are Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotic domains, while Eukarya includes all organisms with nuclei, such as plants, animals, fungi, and many unicellular organisms.

Protista is not a domain because it’s an old, catch-all grouping used for various unicellular and some simple multicellular eukaryotes that don’t fit neatly into plants, animals, or fungi. It’s a convenience category within Eukarya, not a separate, distinct evolutionary lineage at the domain level. So the grouping that is not a domain in the three-domain system is Protista.