The correct option is A Rhodospirillum
Anoxygenic photosynthesis is the phototrophic process, where cellular energy i.e., ATP is produced without producing oxygen. Water is therefore not used as an electron donor. There are several groups of bacteria, that undergo anoxygenic photosynthesis such as green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, Rhodospirillum because they do not produce oxygen as a by-product of photosynthesis. This bacterium is used in nitrogen fixation.