Answer:
I believe the answer to be D. Supernatural.
Explanation:
A flashback is a transition to an earlier time in the story, interrupting the chronological order of events. It does happen in "A Christmas Carol", the story to which this question refers, but it does not concern the face of the door knocker.
Setting is the time, place, and social/historical context in which a story takes place.
The tone is an attitude the author takes toward his or her subject, characters, and readers. It conveys emotions or sensations - happy, dreamy, melancholy etc.
We are left with supernatural. Supernatural is, by definition, an event that cannot be explained by nature or science and is assumed to originate from otherworldly sources. In literature, the most common supernatural elements are probably witches and ghosts. That is the case in "A Christmas Carol". What Scrooge sees is a ghost whose face turns into the face of his friend Marley. Throughout the story, Scrooge is visited by ghosts who show him his past, present, and future. Those visions end up softening his heart.
