Which of the following authors is known for their contributions to modernist literature?

The correct answer is Ernest Hemingway, who is widely recognized as one of the key figures in modernist literature. His writing is characterized by its succinct style, focus on themes of alienation and existentialism, and innovative narrative techniques. Hemingway's works often reflect the complexities of human experience and the disillusionment of the post-World War I generation, both hallmarks of modernist literature.

In contrast, the other authors listed are associated with earlier literary movements. Mark Twain, known for his wit and social commentary, is a central figure in American realism rather than modernism. Charles Dickens, while a masterful storyteller and social critic, wrote during the Victorian era and is associated with the realism and social commentary of that period. Leo Tolstoy's works, which often explore moral dilemmas and the human condition, place him in the realm of 19th-century realism, rather than the modernist perspective that Hemingway embodies. Thus, Hemingway stands out among these authors as a pioneer of modernist literature.