Which of the following is regarded as a pioneer in American modern dance?

Frederick Ashton was a prominent British choreographer known for his work with ballet, particularly with the Royal Ballet. While influential in the dance world, his contributions are primarily rooted in classical ballet rather than modern dance, distinguishing him from pioneers like Martha Graham.

Sergei Diaghilev was a Russian ballet impresario best known for founding the Ballets Russes, which significantly influenced ballet in the early 20th century. His focus was on ballet rather than modern dance, and while he was a key figure in dance history, he does not fit the profile of a modern dance pioneer in America.

Madonna is primarily known as a pop singer and cultural icon rather than a dancer or choreographer. While she has incorporated dance into her performances, she has not made significant contributions to the field of modern dance, unlike Martha Graham.

Maria Tallchief was a celebrated ballerina and the first Native American woman to achieve fame in ballet. Although she made remarkable contributions to dance, her work was primarily in the ballet genre, not modern dance, which differentiates her from pioneers like Martha Graham.