Rhizobium a. fix nitrogen inside nodules on the roots of legumes. b. resemble fungi. c. produce antibiotics. d. produce a gall in plants.

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Option A, fix nitrogen inside nodules on the roots of legumes

Explanation:

Rizhobium bacteria live in symbiotic relationship with roots of leguminous plants such as pulses including alfalfa, soybean etc.

Rhizobium or Bradyrhizobium  is a gram negative bacteria that satisfies the nitrogen starvation of host plant.  

These bacteria get habitation and nutrition from these leguminous plants and in return they fix the nitrogen into the nodules of the roots. This is a kind of facultative association between soil bacteria rizhobium and legume plants.  

Hence, option A is correct

Thus, option A is correct