Which Interaction Would Be Considered a Mutualism

Which interaction would be considered a mutualism?

A) A bumble bee pollinates a tomato plant and receives pollen in return.
B) A cattle egret in a pasture feeds on insects stirred up by cattle hooves.
C) A deer tick drops from a deer and is later picked up by a passing dog.
D) Introduced lionfish in the Caribbean feed on the same small fish as native fish do.
E) The excrement of a flock of birds nesting in a tree kills vegetation under the tree.