A nitrogen fixing microbe associated with Azolla in rice fields is
(A) Frankia
(B) Nostoc
(C) Rhizobium
(D) Anabaena

Hint:Nitrogen fixing microbes have the ability to fix the nitrogen in any part of the plants. Azolla is an aquatic plant which is a free floating plant. These are the freshwater pteridophytes which are commonly called by the name fern.

Complete answer:
The Anabaena azollae has the ability to fix the nitrogen in the plants. They have the nitrogen enzyme for the fixation of the nitrogen. It is the blue green algae which is called cyanobacterium and is free living. These bacteria live in the cavities of the azolla plant. They have the symbiotic relationship with the azolla.

It gives nitrogen to the plant in return the plant provides the nutrients for its growth and development. When the azolla plant is grown along with the paddy plant in the field, these azolla having anabaena in them provide more nitrogen to the paddy plants. This resulted in the fifty percent increase in the yield of the rice. Hence a nitrogen fixing microbe associated with Azolla in rice fields is Anabaena.

Thus, the option (D) is the correct answer.

Note:Like anabaena, Rhizobium also fixes the nitrogen in the mycorrhizae plants. These plants have the special type of roots called haustoria. Rhizobium bacteria live as a symbiotic relationship and possess nif genes in it to fix the nitrogen in these plants' roots.