Which of the following composers is best known as a leading figure of the 20th century's impressionist music?

The correct answer identifies Claude Debussy as a leading figure of 20th-century impressionist music. Debussy's compositional style is characterized by its focus on atmosphere and tone color, rather than traditional forms and structures. He often employed innovative harmonic practices, such as modal scales and unresolved dissonances, to evoke certain moods or imagery in his music, which is a hallmark of impressionism.

Debussy's works, such as "Clair de Lune" and "La Mer," showcase his ability to create a sense of place and emotion through sound, aligning perfectly with the principles of impressionism that emphasize subjective experience and visual representation in music. His influence is profound, as many composers following him were inspired by his techniques and the way he redefined musical norms.

In contrast, the other composers mentioned are associated with different movements or styles in music. For example, Arnold Schoenberg is known for his development of the twelve-tone technique and atonality rather than impressionism. Pierre Boulez is recognized for his avant-garde approaches and serialism. Lastly, Luigi Nono was a prominent figure in post-World War II avant-garde music, focusing on political themes and new forms of expression. Each of these individuals contributed significantly to the evolution of music