Concept:Phrasal verb “broke into” means to suddenly start doing something.Explanation:The sentence indicates that the person suddenly began singing a song upon receiving the promotion letter.“Break into” is used with actions like a song, laughter, or a run, meaning to start suddenly.The other options do not fit this meaning: “broke out” means escaped or started suddenly (for wars/fires), “broke for” means paused for, and “broke in” means interrupted or entered forcibly.Answer:A. broke into