Final answer:
The American playwright known for the play 'The Hairy Ape', and whose work ranged from naturalism to expressionism, is Eugene O'Neill.
Step-by-step explanation:
The foremost American playwright, whose ventures into naturalism eventually turned toward expressionism and who authored The Hairy Ape, was Eugene O'Neill. O'Neill's exploration of drama as an influential art form earned him multiple Pulitzer Prizes and the Nobel Prize for Literature. His play The Hairy Ape embodies the expressionist movement that he came to embrace after his initial period of realism.
O'Neill's influence and versatility in American theater is evidenced by his diverse writing stages and the range of topics covered in his plays, from deep psychological conflicts to societal critiques.