Correct Option: A

Besides cyanobacteria, all the others named were typically sharing the common feature belonging to bacteria domain – i.e. oxygen intolerance. However, cyanobacterium is a phylum which obtains energy through photosynthesis by performing oxygenic photosynthesis and reducing the atmosphere into an oxidising one. The ability to undergo oxygenic reactions distinctly distinguishes cyanobacteria from other bacterial domains. Thereby, cyanobacteria play a very important role in initiating biodiversity and are believed to evolve into plant chloroplast and eukaryotic algae by the process of endosymbiosis. Cyanobacterium due to its stand-out property of oxygenic photosynthesis in an oxygenic environment is thus distinct from other preferably an-oxygenic bacterial domains. ​