Final answer:
'Rhapsody in Blue' is the mixed-style piece that was an important antecedent to 'Porgy and Bess'.
Step-by-step explanation:
The mixed-style piece that was an important antecedent to 'Porgy and Bess' is 'Rhapsody in Blue' (option A).
'Rhapsody in Blue' is a composition by George Gershwin, a prominent American composer of the 20th century. This piece, composed in 1924, combines elements of classical music with jazz, creating a unique hybrid style.
'Porgy and Bess' is an opera composed by Gershwin in 1935. It incorporates elements of jazz, blues, and folk music, much like 'Rhapsody in Blue' did.