Final answer:
Scott Joplin was awarded the Pulitzer Prize posthumously for his opera 'Treemonisha.' Other well-known pieces include 'The Entertainer' and 'Maple Leaf Rag,' which contributed to the spread of ragtime music.
Step-by-step explanation:
Scott Joplin, an influential African American composer and known as the 'King of Ragtime,' won the Pulitzer Prize posthumously for his opera 'Treemonisha.' Although Joplin was highly regarded for his famous ragtime compositions such as 'The Entertainer' and 'Maple Leaf Rag,' it was 'Treemonisha' that garnered this prestigious award after his death. This opera is a significant work because it encapsulates the height of Joplin's musical achievements and contributes to his lasting legacy in American music.