Azolla enriches rice fields with nitrogen due to its association with.
A. Anabaena
B. Nostoc
C. Rhizobium
D. Frankia

Hint: Azolla is an aquatic water-borne fern, found in temperate climate appropriate for paddy cultivation. The fern appears like a green mat over water. The Blue Green Algae cyanobacteria (Anabaena azollae) present as a symbiont with this fern in the lower cavities actually fix atmospheric nitrogen.

Complete step-by-step answer:
Azolla is the most used green manure for rice crops, because of its high growth rate, nitrogen-fixing capability, and ability to trap nutrients from water and soil. Azolla is a floating pteridophyte, containing an endosymbiont of the nitrogen-fixing cyanobacterium Anabaena. It is widely grown in Asian areas; Azolla is either integrated into the soil before rice transplanting or cultivated as a dual crop along with rice. Though Rhizobium is related to the roots of legume plants.
Nostoc is not an alien lifeform, nor is it an alga, plant, or bacterium. Rather then, Nostoc is a cyanobacterium. Cyanobacteria are like bacteria in that they are microscopic, single-celled organisms that do not contain cell nuclei. Nostoc cyanobacteria form single-celled threadlike structures known as a filament.
 Rhizobium is a bacterium available in soil that assists in fixing nitrogen in leguminous plants. It connects to the roots of the leguminous plant and creates nodules. These nodules fix atmospheric nitrogen and turn it into ammonia that can be used by the plant for its growth and evolution.
Frankia supplies most or all of the host plant’s nitrogen needs without added nitrogen and therefore can create a nitrogen-fixing symbiosis with host plants in which nitrogen is the limiting factor in the development of the host. Thus, actinorhizal plants colonize and often prosper in soils that are low in mixed nitrogen.
Therefore the correct answer is Option A.
Note: Anabaena, a genus of nitrogen-fixing blue-green algae with barrel-like or beadlike cells and interspersed extended spores (heterocysts), present as plankton in shallow water and on damp soil. There are both solitary and colonial types, the latter looks like a closely related genus, Nostoc.