Answer: dramatic irony
Explanation: If the author of "The Gift of the Magi", O. Henry, would have made the reader aware of the gift that Jim had purchased in the beginning of the story, this would be an example of dramatic irony. Dramatic irony is a literary device that the author uses to let the audience or the reader know something that the characters in the story do not know. In this case, we were not told what Jim's gift for Della would be. We only know that Della is going to sell her hair to buy a gift for Jim.