Choose the organism that fixes nitrogen in symbiotic association with a water fern.

• The natural organic process that converts atmospheric nitrogen into the nitrogenous composites of living organisms is named symbiotic organic process.
• For case- Rhizobium, Frankia, etc.
• Cyanobacteria (Anabaena azollae) symbiotic relationship. Anabaena lives in leaf pockets of Azolla.

Hence, option D is the correct answer.