Final answer:
Virgil Thomson, not mentioned in the provided options, was the first major American concert music composer who wrote for films, most notably composing the score for the documentary 'The Plow That Broke the Plains' (1936). The provided options, which include Leonard Bernstein, Aaron Copland, John Williams, and George Gershwin, have contributed significantly to music, but are not the correct answer to the question as posed.
Step-by-step explanation:
The first major American composer of concert music to write for films, who composed the scores for several government-sponsored documentary films including The Plow That Broke the Plains (1936), was Virgil Thomson. This is not one of the options provided in the multiple-choice answers, which suggests there might be an error in the question since none of the composers listed are known specifically for that achievement.
It's worth noting that each composer listed had significant contributions to music: Leonard Bernstein is known for the score of West Side Story, Aaron Copland for Appalachian Spring, John Williams for numerous film scores including Star Wars and Indiana Jones, and George Gershwin for Rhapsody in Blue and other works that bridged the gap between concert music and jazz.