The correct answer is A. Blue-green algae.
Blue-green algae are a type of bacteria that can fix nitrogen from the air, making it available to plants. This makes them a valuable biofertilizer for paddy crops, which are nitrogen-hungry plants.
Rhizobium species are bacteria that form a symbiotic relationship with legumes, such as soybeans. These bacteria fix nitrogen from the air and make it available to the legume plant. However, they do not fix nitrogen in the absence of a legume host.
Fungal root fungal are a type of fungus that can form a symbiotic relationship with plants. These fungi can help plants to absorb nutrients from the soil, including nitrogen. However, they do not fix nitrogen from the air.
Azetobactor species are bacteria that can fix nitrogen from the air. However, they are not as effective at fixing nitrogen as blue-green algae. Additionally, they are not as well-suited to use in paddy fields, as they are sensitive to changes in pH and temperature.
Therefore, the best biofertilizer for paddy crops is blue-green algae.