Final answer:
Operetta, vaudeville, and minstrel shows are all sources of the American musical, but the revue is not typically considered a direct source of this genre.
Step-by-step explanation:
The American musical has several sources from which it has evolved, and the question at hand is "Which of the following is not a source of the American musical? A. The revue B. The minstrel show C. Operetta D. Vaudeville".
Vaudeville shows were a type of variety show featuring sketch comedy, music, and sometimes burlesque dancers, making vaudeville a significant influence on the formation of Musical Theatre.
The minstrel show, despite its controversial history, is also acknowledged as a precursor to the American musical, with African American performers contributing to the evolution of the genre.
Operetta was a light form of opera that was popular during the 19th century and also played a role in the development of musicals. However, the revue is not typically mentioned as a direct source of American musicals.