Nitrogen-fixers are the critical links between atmospheric nitrogen and organisms that need nitrogen to survive.
Nitrogen fixers are the compounds which are used in the nitrogen fixation process to convert the nitrogen molecule in other nitrogenous compounds.
So, the nitrogen fixers converts the atmospheric nitrogen molecule (N₂) into the other compounds like ammonia (NH₃) or any other useful nitrogenous compounds which will be used or absorbed by the soil or any other aquatic species. Some examples of nitrogen fixers are bacteria, clostridium, rhizobium, and many more.
Hence option (A) is correct i.e. atmospheric nitrogen and organisms that need nitrogen to survive.
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