Final answer:
Montag reveals his secret stash of books to Mildred, who reacts with shock and disapproval, reflecting her conformity to their book-burning society in 'Fahrenheit 451'.
Step-by-step explanation:
Montag reveals to Mildred that he has been secretly hiding books, a forbidden act in their dystopian society where books are banned and are to be burned by firefighters like Montag. The correct answer to what Montag reveals when he opens the ventilator grill of their air-conditioning system is: b) He reveals his love for books. As for Mildred's response, she is shocked and disapproving, showing a deep indoctrination into the society's norms that view books and the knowledge they contain as dangerous. This incident is integral to the plot of Ray Bradbury's novel 'Fahrenheit 451', showcasing the tension between individual thought and societal conformity.