The american poet who pioneered free verse was william cullen bryant edgar allan poe walt whitman emily dickinson

The correct answer is C. Walt Whitman

Explanation:

Walt Whitman was an American poet and journalist that lived during the 19th century, he is mainly recognized for mixing different movements especially the transcendentalism and the realism in his works. Additionally, Whitman is called the "father of blank verse" because  Whitman was one of the first poets that pioneered free verse (poetry in which meter or rhyme do not follow a specific pattern) as his works did not follow the traditional patterns of meter and rhyme, but instead were similar to prose and used multiple and complex symbols and images. Thus, the poet who pioneered free verse was  Walt Whitman.