The main idea is what the author is mainly thinking about and a central idea is kind of a side idea.
Your answer I believe would be A. The narrator changed what would normally happen in a story. A normal scenario the princess would be frightened by the dragon and the prince would rescue her. However that's not what happened in this story. In this story the princess befriends the dragon. Hope this helps :)
Mood is the overall feeling that the details in a description add up to
Pebble? That would be my guess. I hope I helped! :)