Introduction:
Nitrogen fixation is the process by which atmospheric nitrogen is converted into a usable form by living organisms. In rice fields, Azolla, a small aquatic fern, forms a symbiotic relationship with a nitrogen-fixing microbe. This microbe helps in providing nitrogen to the rice plants, enhancing their growth and productivity. Among the given options, the correct answer is option 'B', which is Anabaena.

Anabaena:
Anabaena is a species of cyanobacteria, commonly found in aquatic environments, including rice fields. It is known for its ability to fix atmospheric nitrogen and form symbiotic associations with plants such as Azolla. Anabaena forms specialized structures called heterocysts, which are responsible for nitrogen fixation.

Symbiotic relationship with Azolla:
Azolla is a small floating fern that grows in rice fields. It forms a symbiotic relationship with Anabaena, where the fern provides a suitable environment for the cyanobacteria to grow, and in return, the cyanobacteria fix nitrogen for the fern and surrounding plants.

Process of nitrogen fixation:
1. Azolla provides a suitable habitat for Anabaena by providing a floating mat on which the cyanobacteria can grow.
2. Anabaena colonizes the cavities present in the Azolla leaves and forms a symbiotic association.
3. Inside the heterocysts of Anabaena, nitrogenase enzymes are produced, which are responsible for the conversion of atmospheric nitrogen into ammonia.
4. The ammonia produced by Anabaena is then transported to the plant cells, where it is assimilated into amino acids and proteins.
5. The plant benefits from the fixed nitrogen, which enhances its growth and productivity.

Advantages of Azolla-Anabaena symbiosis:
1. Azolla acts as a natural biofertilizer, providing a sustainable source of nitrogen to the rice plants.
2. The nitrogen fixation by Anabaena reduces or eliminates the need for synthetic nitrogen fertilizers, reducing environmental pollution and cost.
3. The presence of Azolla also helps in suppressing weed growth in rice fields.
4. The dense mats of Azolla provide shade, reducing water evaporation and maintaining the moisture level in rice fields.

Conclusion:
Anabaena, a species of cyanobacteria, is the nitrogen-fixing microbe associated with Azolla in rice fields. This symbiotic relationship provides a sustainable source of nitrogen for the rice plants, enhancing their growth and productivity. The use of Azolla and Anabaena in rice cultivation can contribute to sustainable agriculture and reduce the environmental impact of synthetic nitrogen fertilizers.