An interaction between two organisms in which one usually benefits is known as

Symbiotic relationship /Interaction  

In a symbiotic relationship, two organisms live in close association and at least one of them is benefited out if this association.  

There are basically three types of symbiotic relationships –

a)Mutualistic Relationship – In this both the organisms living in association get benefited.  

b)Commensalism Relationship – In this association, one organism is benefited while the other is neither benefited nor harmed

c)Parasitic Relationship – In this association, one organism is benefited while the other one is harmed.  

i believe the answer would be b. hope it : )