Final answer:
Aaron Copland studied composition with Nadia Boulanger in France in 1921, a significant phase in his development as a leading American composer.
Step-by-step explanation:
In 1921, Aaron Copland went to France where he was the first American to study composition with Nadia Boulanger. Boulanger was one of the most respected composition teachers of the 20th century, famously known for her rigorous training methods and her ability to inspire her students. Copland's studies with Boulanger marked a significant phase in his development as a composer, contributing to his emergence as one of the leading American composers of his time.