Final answer:
Nadia Boulanger believed George Gershwin didn't need lessons because he already possessed a unique musical voice that could be hindered by formal European-style teaching.
Step-by-step explanation:
Nadia Boulanger rejected George Gershwin not because of any deficiency in his musical abilities, but rather because she believed he already had a distinct compositional voice that did not require formal lessons. Boulanger recognized Gershwin's unique talent in incorporating American jazz elements into classical forms and believed that traditional European-style teaching could potentially hinder his natural style. Although she rejected many other American composers, it was not because she only taught European composers or disliked his style, but because she felt that Gershwin already had what it took to succeed on his own.
Answer: B) She believed he didn't need lessons