Answer: Mutualism
Step-by-step explanation:
Mutualism is the ecological interaction in which two species interact with each other and both the species receive the benefit. Here, the plant species and nitrogen fixing bacteria Rhizobacteria both are receiving benefits of association. The Rhizobacteria is producing IAA for plant and the plant is proving carbon containing molecules to the bacteria which is utilized by the bacteria for growth and colonization.
One advantage of the bacteria producing IAA is that it is the essential hormone for the plant growth. It ensures growth and development processes in plant. It is involved in cell differentiation, elongation, response towards light and gravity.