This is an example of foreshadowing. This is when the plot gives you details, so in this case the nightmare, that symbolizes a dreadful event will take place sometime later in the story. Foreshadowing is a technique used to hint at something that will transpire in the story and the foreshadowing nearly always appears at the beginning of the story.
The answer is B. Aunt Marth has both a kind and cruel personality.
The allusion to Robert Louis Stevenson’s novel Dr. Jekyll and Mr. Hyde in this passage means that Aunt Martha has a personality similar to the "bipolarity" of Dr. Jekyll.
