The passage ends with a statement about reduction of harm to consumers being the primary purpose of antitrust laws. The author makes a point of saying that monopoly power in and of itself is not necessarily harmful. This is supported by paragraph three, in which the firms' monopoly power actually results in lower prices than its customers would otherwise have access to. A offers the best completion of the passage, because it expounds on this point.
Let's go through the rest of the answer choices.
B. This one contradicts what the passage has just concluded. The focus is on abuse of monopoly power, not on monopoly itself. Why would the passage suddenly attack monopoly itself with the last sentence?
C. It doesn't really make sense to return to the specific example of computers and communications. This was used as an example of a tying arrangement in the previous paragraph. We want a more general conclusion to close out the passage.
D. "Supracompetitive prices, however, do not themselves constitute an abuse of monopoly power" (line 31). Again, this is not the problem here.
E. Monopolies are not restrained by the market, but rather by antitrust laws, which allow the monopoly to exist, but not to abuse.
