Final answer:
The first great American composer of the twentieth century was Scott Joplin, who was known for creating ragtime music.
Step-by-step explanation:
The first great American composer of the twentieth century is considered to be Scott Joplin. Joplin was an African-American composer who was born in Texas and traveled throughout the South, living and teaching music in various states as well as Northern cities. He is best known for his role in creating and popularizing ragtime music, a genre that blended European musical traditions with African-American musical influences. Joplin's compositions, such as his famous piece 'Maple Leaf Rag,' had a significant impact on American music and helped pave the way for future American composers.