The correct answer is option C, that is, bacteria.
Lightning produces a comparatively minute concentration of ammonia. However, the foremost transformation of nitrogen to ammonia is done by the microorganisms in the process known as nitrogen fixation.
All the nitrogen-fixing species are bacteria. Some of them live unconventionally from other organisms, thus also known as free-living nitrogen-fixing bacteria. Others live in close symbiotic relationships with plants or other species. The examples of nitrogen-fixing bacteria are Azotobacter and Cyanobacteria.