Nitrogen fixation is the step of the nitrogen cycle that occurs with the help of microbes. Bacteria transform the nitrogen into a usable form for the plants. Thus, option a is correct.
The nitrogen cycle is the part of the biogeochemical cycle that converts nitrogen into other forms that can be used by plants. The nitrogen is converted through the nitrogen-fixing bacterias.
The nitrogen-fixing bacterias are heterotrophic in nature that lives freely in the soil. A few examples are Bacillus, Azotobacter, Clostridium, etc. These bacterias transform the atmospheric nitrogen into usable compounds called ammonia which is the fixed nitrogen and can be used by the plants.
Therefore, nitrogen-fixing bacterias transform nitrogen.
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