In Fahrenheit 451 (Part III: Burning Bright), who most likely turned in Montag and why?
Montag has been pretty careless in his actions throughout the course of the novel; the only time that he is guarded is with Beatty, and for good reason. But around Mildred, and around her friends, he pretty much lets his disillusionment and bitterness at their society show. He even gets out all of his books and forces a very reluctant and disgruntled Mildred to read with him for hours. She doesn't want to--she is afraid of Beatty and the hound, who they suspect has been sniffing around the door, and she finds the books boring and confusing. Later, Montag even goes out and chats with her friends, and has the indiscretion to read them a poem. He gets mad at them, quotes a poem, makes her friends cry, and shouts them out of the house. Such bizarre behavior does not go unnoticed in Montag's society, especially if it involves literature and books.
So, when they show up at the house, Montag is shocked at first, but, the more he thinks about it, he suspects that "Mildred, of course" was the one who put the alarm in. His own wife. Beatty confirms this when he nods when Montag asks, and then he adds additional information: "But her friends turned in an alarm earlier that I let ride." So, Montag had actually been reported a couple times, first by Mildred's friends, and secondly by Mildred herself. It is understandable why her friends did, but Mildred is a bit hard to swallow--his own wife. It's a sad situation for Montag, but it does prompt him to greater rebellion and independence. I hope that those thoughts helped; good luck!