Assertion- Azolla is used as a biofertilizer in rice fields.
Reason- Azolla contains nitrogen-fixing cyanobacteria in its leaf cavities.
A. Both Assertion and Reason are correct and the reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but the reason is not the correct explanation for Assertion
C. The assertion is correct but the reason is incorrect
D. Both Assertion and Reason are incorrect

Hint: Azolla is a floating pteridophyte that is utilized as a biofertilizer in the rice fields. It is incorporated into the soil before the process of rice plantation or seeded as a double plantation along with the rice plants. This is wielded as the biofertilizer as it comprises of a nitrogen-fixing bacteria known as Anabaena azollae.

Complete answer:
The free-floating marine fern Azolla is said to be small as well as heterosporic, with a widespread percentage in quiet waters such as rivers, dams, creeks, etc., and is deemed as an invasive species. This is the only known fern with a lasting symbiotic association with the heterocysts nitrogen-fixing cyanobacterium Anabaena azollae. It is where the cyanobiont is conveyed through the Azolla productions without a de novo infection.
Azolla is a free-living, aquatic, nitrogen-fixing blue-green algae. Nitrogen fixation is the process in which the molecular nitrogen present in air converts into ammonia, with the help of microorganisms such as bacteria. Nitrogen-fixing bacteria are prokaryotic organisms that convert nitrogen into ammonia, which is then used by the plants. There are two types of nitrogen-fixing bacteria:- Symbiotic and Non-symbiotic. Some of the Symbiotic bacteria are Azospirillum species and Rhizobium and Frankia. The Non-symbiotic bacteria include Cyanobacteria, Anabaena, and
Nostoc.
Hence, the correct answer is option A.

Note: The cyanobiont along with other bacteria genera inhabit an ovoid pocket in each dorsal lobe of the coating. The cyanobiont has a high rate of nitrogen fixation as well as thus this symbiosis was evaluated heeding its bio fertilization.