The correct answer is cyanobacteria.Concept:Bacteria: These are microscopic organisms and the sole members of Kingdom Monera.They are unicellular prokaryotes.Explanation:Cyanobacteria:They are also known as Blue-green alga.They have a green pigment similar to chlorophyll a.They are autotrophic in nature i.e., they can make their own food.They are unicellular, colonial, or filamentous. The colonies are generally surrounded by a gelatinous sheath.They often form blooms in polluted water bodiesSome of these organisms can fix atmospheric nitrogen in specialized cells called heterocysts, e.g., Nostoc and Anabaena. Additional InformationMycoplasma: These are the smallest bacteria known that can survive without oxygen and lacks a cell wall.Diatoms: They are unicellular eukaryotes that belong to the kingdom Protista. They are the chief producers of the oceans.