D. To make sure that Banquo's son won't become king as the witches predicted
Explanation:
Banquo appears in the play as an equal to Macbeth. They both are very brave soldiers and capable men. However, when the two men listen to the witches' prophecies, Banquo discovers that his children will become the rulers of Scotland. Macbeth decides to kill Banquo and his son, Fleance, to prevent the prophecy from occurring. However, he is able to kill Banquo but his son escapes, indicating the prophecy will be fulfilled.
