The correct answer is B) foreshadowing.
The nightmare is an example of a foreshadowing.
King Arthur has a nightmare at the beginning of the plot. In that nightmare, he sits above “hideous deep black water”, and then falls into a pit of snakes and monsters. What King Arthur dreamt can be interpreted as a foreshadowing.
Foreshadowing is a term that means that we get a sign in advance to what is coming. It could be a warning of what is coming in the future. When writing a story, the author can include foreshadowing in some passage to catch the reader attention on something that is going to occur in the chapters coming.