Correct Option: D

The nitrogen-fixing capability of Azolla has led to it being widely used as a biofertiliser for increasing the productivity of rice fields. When rice paddies are flooded in the spring, they can be inoculated with Azolla, which then quickly multiplies to cover the water, suppressing weeds. The rotting plant material releases nitrogen to the rice plants, increasing the rice yield equivalent to that produced by 30-60 kg N/ha.