Final answer:
In 'Fahrenheit 451', Mildred, Montag's wife, is the informant that causes Captain Beatty to investigate Montag's house.
Step-by-step explanation:
The informant that prompted Captain Beatty to investigate Montag's house was 1) Mildred, Montag's wife. In Ray Bradbury's novel 'Fahrenheit 451', Mildred becomes overwhelmed by the situation surrounding her husband's secret collection of books. Out of panic and conformity to the society's anti-book stance, she informs the authorities about Montag's hidden books, leading to Captain Beatty's investigation and the ultimate burning of their house.