Zoology - Microbes in Human Welfare Question with Solution | TestHub

${NH}_4\stackrel{\mathrm{N}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{o}\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{s}}{\to }{NO}_2\mathrm{ }\stackrel{\mathrm{N}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{r}}{\to }{NO}_3$.

Azotobacter is a non-symbiotic biofertilizer. They are aerobic, free-living soil microbes. They play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen which is inaccessible to plants. They release it in the form of ammonium ions into the soil. Therefore, they are used by humans for the production of biofertilizers.

Rest of the options are symbiotic.