Question

what is the name of the fern that grows in rice paddies and provides a source of nitrogen fertilizer to the rice plants?
salvinia
selaginella
azolla
amorpha

Explanation:

Azolla is a fern that forms a symbiotic relationship with nitrogen - fixing cyanobacteria. It grows in rice paddies and provides nitrogen to rice plants. Salvinia is a floating fern but not known for nitrogen - fixing in rice paddies. Selaginella is a spike - moss, not a nitrogen - providing fern for rice. Amorpha is a leguminous plant, not a fern.

Answer:

C. Azolla