For paddy, the best fertilizer is

The correct option is Azolla pinnata.

Explanation:

Paddy (rice) fields require a continuous supply of nitrogen for healthy growth and high yields. Azolla pinnata is a species of tiny freshwater fern that hosts a symbiotic blue-green alga (cyanobacterium) called Anabaena azollae within its leaf cavities.

This cyanobacterium has the unique ability to fix atmospheric nitrogen ($N_2$) into a bioavailable form (ammonia) that the paddy plants can easily absorb. As the fern grows and eventually decomposes in the flooded paddy fields, it releases this fixed nitrogen, acting as an excellent, eco-friendly biofertilizer.

Let us briefly look at why the other options are not the primary biofertilizers for paddy:

• Bacillus polymyxa and Bacillus megaterium: These are soil bacteria known for phosphate solubilization and promoting general plant growth, but they are not the primary nitrogen-fixing biofertilizers specifically associated with flooded paddy fields.

• Rhizobium meliloti: Rhizobium species form symbiotic relationships with the root nodules of leguminous plants (such as alfalfa or clover) to fix nitrogen, and they are not suitable for non-leguminous cereal crops like paddy (rice).