Concept:Mutualism is a close association in which both organisms benefit from each other.Explanation:Bacteria such as Rhizobium live inside the root nodules of leguminous plants.The bacteria convert atmospheric nitrogen into nitrates that the plant can absorb and use.In return, the plant supplies the bacteria with shelter and carbohydrates produced during photosynthesis.Since both the bacteria and the plant gain a benefit, this type of association is mutualism.Commensalism, parasitism, and saprophytism do not describe this mutually beneficial relationship.Answer:B. mutualism.