The correct option is C Bacteria
Free-living and symbiotic nitrogen-fixing bacteria fix atmospheric inert nitrogen into biologically useful ammonia thereby adding nitrogen to the soil and increasing its fertility. Algae increase the organic carbon content of the soil upon their death. Mycorrhizae are the symbiotic fungi that reside in roots of higher plants and increase soil fertility by nitrogen fixation. Phages are the bacterial viruses that replicate within the bacterial cell with a polyhedral head, a helical tail and fibers for attachment to the host cell. Thus, the correct answer is option C.