Final answer:
The organism described in the question, which is unicellular, has no nucleus, and can live in harsh environments, belongs to the domain Archaea (option A).
Step-by-step explanation:
The organism described in the question, which is unicellular, has no nucleus, and can live in harsh environments, belongs to the domain Archaea (option A).
Archaea are one of the three domains of life and consist of prokaryotic organisms. They have no nuclei and are single-celled. Archaea are known for their ability to survive in extreme and harsh environments, including hot springs, acidic environments, and deep-sea hydrothermal vents.