Unconscious would be the best synonym for subliminal
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.
Do you have more of the sentence or is that it?
Personally, I don’t see anything wrong with your sentence in the first place. All necessary punctuation was in your beginning sentence making “D” the correct answer to your problem. Hope this helps you :)