Question

Sol. Key concept: Nitrogen-fixing cyanobacteria form a mutualistic association with Azolla.

The traditional textbook name of the symbiont is Anabaena azollae. It lives in cavities in the leaves of the floating water fern Azolla. Its specialised cells, called heterocysts, provide conditions suitable for fixation of atmospheric nitrogen.

The cyanobacterium supplies biologically usable nitrogen, while the fern provides a sheltered habitat and resources. This association is the reason Azolla is used as a biofertiliser, particularly in rice cultivation.

Anabaena is therefore the required option b. Although older texts call it a blue-green alga, it is a prokaryotic cyanobacterium, not a eukaryotic alga.