The significance of Fleance escaping lies primarily in option b: The prophecy of the witches cannot come true if he dies.
Fleance's survival means that the witches' prophecy, which foretells that Banquo's descendants will inherit the throne, remains possible. His escape ensures that the potential continuation of Banquo's lineage—and thereby the fulfillment of the witches' prophecy—remains intact. This adds tension to the narrative and deepens the themes of ambition and fate throughout "Macbeth."