Final answer:
Benny Goodman's trio transitioned into a quartet with the addition of the African American vibraphonist Lionel Hampton, signifying a culturally dynamic and racially integrative period in jazz history.
Step-by-step explanation:
Benny Goodman's trio became a quartet with the addition of Lionel Hampton, who played the vibraphone. Goodman's group was hailed for its innovative sound and for the racial barrier-breaking integration when Hampton, an African American musician, joined the previously all-white ensemble. The Benny Goodman Quartet was known for its contributions to the genre of jazz, which was undergoing a significant transformation during the 20th century. Jazz, as a music form, faithfully reflected the cultural dynamism of America and significantly African American communities, with its rhythm, improvisation, and bold harmony.