Which composer is categorized in the 20th/21st-century period?

Prokofiev is classified as a composer of the 20th/21st century due to his significant contributions to music during this period, particularly with his innovative compositions that reflect the evolving musical styles and trends of the time. His works, such as "Peter and the Wolf" and his numerous symphonies, operas, and ballets, showcase the integration of modern harmonies, rhythmic complexities, and diverse influences that emerged in the 20th century.

In contrast, Bach, Haydn, and Mozart were pivotal figures in earlier periods: Bach was part of the Baroque era, known for his intricate counterpoint; Haydn was a key composer in the Classical period, recognized for developing symphonic forms and sonata-allegro structure; and Mozart, also a Classical era composer, is celebrated for his melodic genius and operatic contributions. Therefore, while the others made enduring contributions to music, they do not fit into the classification of the 20th/21st-century period like Prokofiev does.