Final answer:
The smallest free-living organisms, known as prokaryotes, include bacteria and archaea. Prokaryotes are single-celled organisms without nuclei or other membrane-bound organelles.
Step-by-step explanation:
The group of bacteria considered to be the smallest free-living organisms is prokaryotes. Bacteria and Archaea are both examples of prokaryotes, which are single-celled organisms lacking intracellular organelles. Unlike eukaryotes, prokaryotes have no nuclei or other membrane-bound organelles.