Final answer:
The 20th-century composer who pushed the boundaries of musical roles was Igor Stravinsky.
Step-by-step explanation:
The 20th-century Russian and later French and American composer and pianist who was a very important and influential composer, pushing the boundaries of musical roles, was Igor Stravinsky. Stravinsky was known for his innovative and groundbreaking compositions that challenged traditional musical conventions.
One of his most famous works is The Rite of Spring, a ballet that scandalized audiences with its unconventional rhythms and dissonant harmonies. Through his experimentation with different musical styles and techniques, Stravinsky left a lasting impact on the development of 20th-century music.