The correct answer is A. Bacteria live in root nodules of a plant, process nitrogen to ammonia for the plant, receive protection and steady food supply which is being got in return.
Mutualism is referred to as the way two organisms which are of different species exist in relationship whereby every individual benefits from activity of each other.
The relationship which is between bacteria and ungulates with the intestine is or good example of mutualism.
Mutualism plays an important role in ecology. We can say that if we measure the exact fitness what we get as a benefit is a mutualistic relationship.