The correct answer is option-2, Mycoplasma. Mycoplasma is a genus of bacteria that are characterized by their lack of a cell wall, making them the smallest known prokaryotic organism. They are unicellular and can be found in various environments, including soil, water, and even in the human body. Mycoplasma are unique in that they have a minimal genome size and a small cell size, allowing them to survive in diverse habitats. This adaptation is possible due to their unique ability to take up nutrients directly from the surrounding environment. Microcystis is a type of cyanobacteria commonly found in freshwater ecosystems known for forming harmful algal blooms. Bacteria, on the other hand, is a broad term that encompasses a wide range of prokaryotic organisms that can vary greatly in size. Chlorella is a genus of green algae and is not a prokaryotic organism.