Correct option is B. Rhizobium
Some bacteria maintain the soil fertility and others enhance it. The fertility of the soil is proportional to the nitrogen content. Nitrogen is an important ingredient in the structure and function of the living beings. Nitrogen-fixing bacteria like Azotobacter, Clostridium live free in the soil and fix atmospheric nitrogen. Rhizobium like bacteria live symbiotically with root systems of leguminous plants and fix from the atmospheric nitrogen. Therefore, the correct answer is option B.