Which type of organism is characterized by having membrane-bound nuclei in its cells?

Eukaryotes are clearly defined by the presence of membrane-bound nuclei in their cells. This cellular architecture is a fundamental characteristic that differentiates them from prokaryotes, which do not have a true nucleus; instead, their genetic material is located in a nucleoid region without membrane separation.

In eukaryotic cells, the nucleus is surrounded by a nuclear membrane, which provides a distinct environment for the storage and replication of DNA, as well as the regulation of gene expression. This complexity is also reflected in eukaryotic organisms, which include all plants, animals, fungi, and some microorganisms. The presence of membrane-bound organelles in addition to the nucleus allows for specialized functions within the cell, further enhancing its capabilities.

The other options represent groups of organisms that do not possess a membrane-bound nucleus; for example, bacteria and archaebacteria, both of which are types of prokaryotes, have simpler cellular structures that lack this defining feature.