I believe the correct answer is D. Cowboy songs.
Aaron Copland, American composer, used cowboy songs to highlight
the frontier themes in his ballets “Billy the Kid” and “Rodeo”. Since Copland’s
most important ballets are based on American folk material (Billy the Kid
(1938), Rodeo (1942), and Appalachian Spring (1944)), he used cowboy songs and
American folk songs to contribute to the main themes.
