In the novel "Fahrenheit 451" by Ray Bradbury, Mildred reacts to Montag's exposing his secret stash of books with option D, she feels angry and scared and wants him to reconcile the situation. Mildred is a conformist and has been brainwashed by the society she lives in, which values entertainment over intellectual pursuits. Therefore, when Montag shows her his hidden books, she becomes fearful because possessing books is illegal and can lead to severe punishment, and she urges him to get rid of them to avoid being caught.