Final answer:
Members of Domain Archaea are unicellular, have cells that lack membrane-bound organelles, and often live in harsh environments, such as deep-sea vents and hot springs. These microorganisms are prokaryotes, meaning they lack a nucleus and membrane-bound organelles.
Step-by-step explanation:
The correct domain for unicellular organisms that lack membrane-bound organelles and often live in harsh environments is Archaea. These organisms, known as extremophiles, are prokaryotes that have a unique genetic makeup distinguishing them from bacteria.
The domain Archaea includes many extremophiles that thrive in extreme conditions, which can include high temperature, salinity, or pressure. Despite superficial similarities to bacteria, members of the Archaea domain are genetically distinct. This discovery, made by analyzing the sequence of small-subunit ribosomal RNA, led to the establishment of the three-domain system of classification, which includes Bacteria, Archaea, and Eukarya.