Anaerobic N-fixing bacteria

Answer: Option A

Nitrogen-fixing bacteria are prokaryotic microorganisms that are capable of transforming nitrogen gas from the atmosphere into fixed nitrogen compounds that can be easily available for plants for absorption like ammonia.
Bacteria can be anaerobic like Clostridium and aerobic like Azatobacter.
Clostridium is a non-symbiotic, anaerobic nitrogen fixing bacteria that fix nitrogen in the absence of oxygen.