Final answer:
The opera that serves as a model for the verismo movement is Carmen. It represented the style of realistic characters, intense emotions, and dramatic settings that influenced the verismo composers.
Step-by-step explanation:
The opera that served as a model for the three Italian composers who led the verismo movement is B) Carmen. Verismo was an Italian operatic movement that aimed to bring the naturalism of influential novels to the opera stage, focusing on the lives of ordinary people. Carmen, by the French composer Georges Bizet, though not an Italian opera, exemplified many of the qualities that would typify verismo, such as realistic characters, intense emotions, and dramatic, everyday settings. It's an error to directly associate Carmen with the movement since it predates verismo, but its style certainly influenced the verismo composers like Puccini, Mascagni, and Leoncavallo in their own creations.