This is an inference question. The passage suggests that, since oxpecker birds are commensal species with large mammals, they benefit from the creatures that are attracted to the mammals' fur, but the mammals themselves are neither helped nor harmed by the relationship. Consider each choice, looking for a reasonable inference:

(A) This choice gets the commensal relationship exactly backwards.
(B) This is clearly wrong. If oxpecker birds feed on ticks, fleas, and flies, clearly the ticks, fleas, and flies are neither benefiting nor neutral in their relationship with the oxpecker birds.
(C) This choice is too extreme. The passage only describes this relationship; it doesn't tell us that it is exclusive.
(D) This is also too extreme. It may be true in some instances, but the three sentences of the passage don't provide enough evidence to reasonably deduce this.
(E) This is correct. It merely restates the definition of commensalism in terms of the role of the mammals in their relationship with oxpecker birds.