Option A, Blue green algae, acts as a bio-fertilizer for the rice crop. Blue-green algae, also known as cyanobacteria, establish a symbiotic relationship with rice plants, providing them with fixed atmospheric nitrogen. This process, known as nitrogen fixation, enhances soil fertility by increasing the availability of nitrogen to the rice plants. The nitrogen-fixing ability of blue-green algae reduces the need for synthetic fertilizers, thereby promoting sustainable agricultural practices. Rhizobium species (Option B) typically form symbiotic relationships with leguminous plants, aiding in nitrogen fixation in their root nodules. Mycelium (Option C) refers to the vegetative part of fungi and does not directly act as a bio-fertilizer for rice crops. Azotobacter (Option D) is a free-living nitrogen-fixing bacterium that can enhance soil fertility but is not commonly associated with rice cultivation as blue-green algae are.