Final answer:
Alfred Stieglitz's signature work is 'The Steerage', a photograph from 1907 that is seen as his first modernist work and an iconic piece in contemporary photography. Option A is correct.
Step-by-step explanation:
Alfred Stieglitz's signature work is considered to be The Steerage, which he captured in 1907 using the photogravure technique. This pivotal photograph is often recognized as his first 'modernist' photograph and stands as an iconic representation of contemporary photography.
The image skillfully depicts the social reality of immigrants in the steerage section of a ship with strong compositional elements, contributing significantly to the perception of photography as a fine art form.
Additionally, Stieglitz's work marked a turning point in art, promoting the idea that photography should be appreciated for its unique qualities instead of mimicking traditional painting.