Final answer:
Archaea is the group of bacteria considered to be the smallest free-living organisms. They are unicellular, prokaryotic microorganisms that differ from bacteria.
Step-by-step explanation:
The group of bacteria considered to be 'the smallest free-living organisms' is known as Archaea. Archaea are unicellular, prokaryotic microorganisms that differ from bacteria in their genetics, biochemistry, and ecology. Some archaea are extremophiles, living in environments with extremely high or low temperatures, or extreme salinity. They do not live in great numbers in human microbiomes and are not known to cause disease.