Which of the following is a living microscopic one-celled organism?

A bacterium is indeed a living microscopic one-celled organism. Bacteria are prokaryotic microorganisms, characterized by their single-celled structure and lack of a distinct nucleus. They can be found in various environments, including soil, water, and within other living organisms. Bacteria play diverse roles in ecosystems, including decomposing organic matter and participating in nutrient cycling.

In contrast, viruses do not possess the characteristics of living organisms since they require a host cell to replicate and cannot carry out metabolic processes independently. Fungi, on the other hand, typically exist as multicellular organisms (like mushrooms) or as single-celled organisms (like yeast), but they are not classified as bacteria. Algae can be unicellular or multicellular and are classified under a different category, being primarily photosynthetic and belonging to the kingdom Protista, which makes them distinct from bacteria.