Final answer:
The Archaea kingdom consists of unicellular, prokaryotic organisms that live in extreme environments, such as thermal vents and salt marshes, known for their extremophile nature.
Step-by-step explanation:
The kingdom that is unicellular, prokaryotic, and lives in extreme environments such as thermal vents and salt marshes is Archaea. Organisms in the domain Archaea can thrive in harsh conditions, such as high temperature, salinity, and pressure, which is why they're found in places like the Morning Glory pool and the deep-sea hydrothermal vents. The members of this kingdom are known as extremophiles due to their ability to live in such unforgiving environments. Despite their appearance similar to bacteria, archaeans are genetically different and belong to their own distinct domain.