Final answer:
Igor Stravinsky is the composer known for pushing the boundaries of musical roles, particularly with his composition 'The Rite of Spring'.
Step-by-step explanation:
The composer known for pushing the boundaries of musical roles is Igor Stravinsky. He was a Russian composer and is best known for his composition called The Rite of Spring, which completely scandalized audiences when it was first performed. Stravinsky's music challenged traditional conventions and his innovative work had a significant impact on the development of 20th-century music.