Which of the following composers is known as a nationalist composer from the U.S.?

The correct answer highlights Aaron Copland, who is recognized as a prominent nationalist composer from the United States. Nationalism in music often involves incorporating elements that are representative of a particular country’s culture, history, and folk traditions. Copland's work embodies American themes and often utilizes folk melodies, rhythms, and musical forms that reflect the American landscape and spirit.

His compositions, such as "Appalachian Spring" and "Fanfare for the Common Man," showcase the distinctive qualities of American life and music, making him a key figure in the nationalist movement within the context of U.S. music.

The other composers listed are notable in their own right but do not align with the specific characterization of nationalism as it pertains to American music. Béla Bartók, for example, is known for his ethnomusicological work in Eastern European folk music, while Edward Elgar is associated with British nationalism. Enrique Granados, on the other hand, is recognized for his contributions to Spanish music and the impressionistic style, not American nationalism.