Azolla is a small aquatic fern that is commonly used as a biofertilizer for rice cultivation. It has a unique symbiotic relationship with a group of nitrogen-fixing cyanobacteria known as Anabaena. Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen into a form that plants can use.

Option 1, Azotobacter, is not the correct answer. Although Azotobacter is a nitrogen-fixing bacterium, it does not have a symbiotic relationship with Azolla.

Option 2, Azospirillum, is also not the correct answer. Azospirillum is another type of nitrogen-fixing bacterium, but it does not form a symbiotic relationship with Azolla.

Option 3, Aulosira, is not the correct answer either. Aulosira is a filamentous cyanobacterium, but it does not have a symbiotic relationship with Azolla.

Therefore, the correct answer is option 4, Anabaena. Anabaena forms symbiotic associations with Azolla, specifically in specialized cavities known as "hairs" on the underside of Azolla`s leaves. Inside these cavities, Anabaena perform nitrogen fixation, providing a natural source of nitrogen to the Azolla plants