Free living nitrogen fixers in soil that helps in N₂ fixation include

Nitrogen-fixing bacteria are microorganisms able to convert atmospheric nitrogen into fixed nitrogen.  
Two types of nitrogen-fixing bacteria have been identified. 

They are:
1. Free (nonsymbiotic) bacteria: cyanobacteria (or blue-green algae), Anabaena and Nostoc and genera like Azotobacter, Beijerinckia and Clostridium.
2. Mutualist bacteria (symbiotic): Rhizobium, associated with legumes.
Thus, the correct answer is option (D).