Final answer:
A bacterium is a prokaryotic microorganism characterized by the lack of a distinct nucleus and membrane-bound organelles.
Step-by-step explanation:
The answer to which of the following is a prokaryotic microorganism is d) Bacterium. Prokaryotic organisms are defined by the absence of a distinct, membrane-bound nucleus and membrane-bound organelles; this describes the cells of bacteria. In contrast, protists, fungi, and algae are all eukaryotic organisms because they have cells that contain a nucleus and organelles. Organisms within the domains of Bacteria and Archaea consist of prokaryotic cells.