I still can not figure out that. Can any NN come to say why the (E) is the best choice? In fact, I don`t think the (E) is more direct than (D). Furthermore, the (C) is like a traditional GMAT answer.
E is best because it reinforces the conclusion. Outsourcing not only saves costs, but also saves managers' effort on uneconomical part, so these effort can be devoted to the part most needed managers' attention.
D just repeats the premise that outsourcing can save costs. C is irrelevant.