Which of the following is a unicellular organism without membrane-bound nuclei?

The correct answer is prokaryote, which refers to a group of organisms that are characterized by the absence of a membrane-bound nucleus and other membrane-bound organelles. Prokaryotes are typically unicellular organisms, with bacteria being the most well-known representatives of this group.

In prokaryotic cells, the genetic material is located in a region of the cell called the nucleoid, which is not surrounded by a membrane. This structural feature differentiates prokaryotes from eukaryotes, which do have a defined nucleus and more complex cellular organization.

While archaebacteria, often incorrectly referred to separately, are a type of prokaryote, they fall under the broader category of prokaryotic organisms. Thus, stating prokaryote encompasses all unicellular organisms that lack a membrane-bound nucleus. The other options, such as eukaryotes and fungi, are multicellular or contain membrane-bound organelles, distinguishing them from the prokaryotic classification.