In my opinion, the correct answer is D. If Macbeth took the crown unjustly. Banquo thinks about Macbeth's ascension to the throne and says "I fear tThou played’st most foully for ’t" - which means that Macbeth probably cheated somehow. However, as much as Banquo suspects Macbeth's intentions and means, he doesn't think that Macbeth would kill him. He doesn't suspect the witches' prophecy either, as it turned out to be true. And that prophecy didn't mention his (Banquo's) getting the crown, so he doesn't mention it. In this monologue, he considers the future and thinks if the prophecy would be fulfilled for him, as it was for Macbeth.
