Assertion (A) : Biological N₂ fixation takes place in anaerobic medium.  
Reason (R) : Nitrogenase enzyme involved in nitrogen fixation is very sensitive to oxygen. 

Biological nitrogen fixation typically occur in anaerobic (oxygen-depleted) media.The reason for this is that the nitrogenase enzyme complex, which is responsible for nitrogen fixation, is highly sensitive to oxygen. Nitrogenase requires an anaerobic (low-oxygen) environment to function properly. When exposed to oxygen, it can become inactivated or inhibited.