Which group of organisms includes single-celled micro-organisms that can tolerate extreme conditions?

The correct choice is Archaea because this group consists of single-celled microorganisms that thrive in extreme environments, such as hot springs, salt lakes, and deep-sea hydrothermal vents. Archaea possess unique biochemical and genetic characteristics that allow them to survive in conditions that would be inhospitable to most other forms of life, including high temperatures, high salinity, and extreme pH levels. They often have specialized enzymes and metabolic pathways that enable them to utilize alternative energy sources, contributing to their resilience in extreme conditions.

In contrast, while Eukarya includes a wide range of organisms, such as plants, animals, and fungi, it does not primarily encompass extremophiles (organisms adapted to extreme conditions). Fungi, also part of Eukarya, are generally not single-celled organisms and are more typically found in less extreme environments. Bacteria do include some extremophiles, but Archaea are specifically known for their ability to survive and thrive in the most inhospitable environments, distinguishing them from both Eukarya and Bacteria.