The roots of leguminous plants show small nodule like structures called as nodular roots. The nodules are inhabited by colonies of rod shaped, nitrogen fixing bacteria called Rhizobium. Normally higher plants cannot utilize atmospheric nitrogen directly as it is diatomic. So this bacterium absorbs atmospheric nitrogen and converts it into ammonia and supplies to the host plant. In return, the bacterium receives food and shelter from the host plant. This type of mutual relationship between two different organisms is called symbiosis. Hence option A is correct.