If you had shared some options it would have been much easier to give you the right answer. According to the general idea of this story, I think that the statement which best explains why Irving sets "The Adventure of the Mysterious Stranger" in a land of “masks and gondolas” is ''The setting is symbolic of the idea that ease and affluence are available to all''. I think you have the same in your option list, and if so, it will definitely help you.
