Hi!
The correct answer would be bacteria.
Step-by-step explanation
Nitrogen fixing, or nitrogen fixation, is the conversion of atmospheric nitrogen into compounds that can be readily utilized by other organisms. These compounds are ammonia and other nitrogenous compounds. Bacteria have the ability to convert the inert nitrogen in the atmosphere into these nitrogen containing compounds, and are predominantly found in a symbiotic relationship with plants, or in the soil. These particular types of bacteria are known as nitrogen-fixing bacteria, such as Rhizobium.
Hope this helps!