Answer:
The correct answer is "if ill, Why hath it given me earnest of success, Commencing in a truth?"
In this part of Macbeth's monologue we can see Macbeth's doubt and uncertainty. He wonders why would the witches lie after telling him the truth. He starts to believe that the second profecy, just like the first one, will come true, and thus starts to consider about commiting the horrible crime that would be necessary for that to occur.