Nitrogen-fixing bacteria are mostly prokaryote. They are capable of transferring atmospheric nitrogen gas to nitrogen fixed compounds like ammonia. They are of two types i.e.free living bacteria that are also known as non-symbiotic bacteria. Example: Cyanobacteria,Clostridium,Rhodospirillum etc. But Rhodospirrillum is facultative anaerobic bacteria.While the second bacteria is known as symbiotic bacteria that shows symbiosis.Example: Rhizobium etc. So the correct answer is D.