Final answer:
The composer who first attracted attention to the idea of third stream, which had not yet been named, was Duke Ellington. He was a key figure in blending jazz with classical music elements, contributing to the birth of the third stream genre.
Step-by-step explanation:
The composer who first attracted attention to the idea of third stream, which had not yet been named, was c)Duke Ellington. Third stream is a musical genre that is a synthesis of classical music and jazz.
Duke Ellington was a prominent jazz composer and band leader whose works displayed a sophisticated melding of the two styles, contributing early to what would be recognized as third stream music.
While composers like Igor Stravinsky and Arnold Schoenberg were known for their avant-garde and modernist approaches to classical music, third stream as a concept looked specifically at merging the techniques and characteristics of classical with the improvisational nature of jazz, something that the other composers listed did not specialize in.
Although John Cage was an avant-garde composer, his work, like the piece 4′33″, focused more on chance procedures and experimental music rather than blending jazz and classical music.
Wolfgang Amadeus Mozart, being an iconic figure from an earlier classical period, could not have contributed to the third stream concept. Hence, Duke Ellington is the correct answer.