Final answer:
Methanogens, halophiles, and thermophiles are all archaea species that thrive in extreme environments.
Step-by-step explanation:
The organisms in the groups methanogens, halophiles, and thermophiles have in common that they are all archaea species. Archaea are microorganisms that have distinct characteristics separating them from bacteria and eukaryotes. Methanogens, halophiles, and thermophiles are all types of archaea that thrive in extreme environments. Methanogens produce methane as a byproduct of their energy metabolism, halophiles live in high-salt environments such as salt flats or the Dead Sea, and thermophiles thrive in high-temperature environments like hot springs or volcanic vents.
Learn more about Similarities among methanogens, halophiles, and thermophiles