Explanation:

Azotobacter

Atmospheric nitrogen is not available to the plants. Nitrogen fixing bacteria converts atmospheric nitrogen into ammonium compounds and make it available to the plants. There are two types of bacteria, free-living and symbiotic. Example of free living nitrogen fixing bacteria include Azotobacter. The example of symbiotic nitrogen fixing bacteria includes Rhizobium. Rhizobium leguminosarum is a symbiotic bacterium which lives in symbiotic association with the legume plants and fixes atmospheric nitrogen.
Thus, the correct answer is 'Rhizobium leguminosarum.'