C) to explain why some people in the south believed in witches.
In this story, we see that Tom and Huck tiptoe through the garden when Jim hears them. He decides to stay put until he discovers the source of the noise, but he ends up falling asleep. Huck and Tom decide to play a prank on him and they put his hat on a branch. When Jim wakes up, he tells everyone that he was the victim of witchcraft. Twain uses this story in order to explain the superstitions of the South and how these allowed people to make sense of the world.
Macduff adds that the chamberlains seem the most likely murderers, and that they may have been paid off by someone to kill Duncan. Suspicion has now fallen on the two princes, Malcolm and Donalbain, because they have fled the scene.