The anwser is A: That like Polybus, the prophecy is dead and buried.
By saying that the prophetic oracles were worthless, the speaker demonstrates his complete dislike and disregard for them. And to emphasize his opinion, he adds that Polybus has taken them to Hades (in older Greek mhyts this the abode of the dead,where all mortals go when they die), which is a place where he already is.
