Macbeth says these lines after he hears the witches' prophecies. He's basically trying to figure out what to do with that information, and in particular he's trying to figure out if it's good or evil. He's all like, "If witches are evil, then why are they telling me such good news?" (They told him he'll be king one day.)
The best answer therefore seems to be D. These are witches, who are presumed to be evil, but they've given him good news and one of their prophecies has already been proven true ("commencing with a truth").
B is also a good answer, but it's less directly supported by these lines, since Macbeth isn't directly addressing the danger of the witches' darkness.
