The literary device that the author is using is dramatic irony. Dramatic irony means when the reader/audience is aware of something a character or characters is not. Symbolism is the use of symbols or pictures to represent an idea, so it will not fit this case. A hyperbole is like exaggerating to make a point, which also doesn't fit this case. Understatement is basically presenting something as being smaller or less of value (minimizing). And description is like a report on something that gives details about it so we can recognize it somehow, which is nowhere in this example/ Therefore, the only answer that makes sense and fits in this situation is Dramatic Irony.
Hope this helps :)