Final answer:
Alfred Stieglitz is the artist who opened a New York gallery exhibiting progressive European art, and also showcased American artists like Georgia O'Keeffe.
Step-by-step explanation:
The artist who opened a New York gallery that exhibited progressive European art was Alfred Stieglitz. As a pioneer in the photography industry and a champion of modern art, Stieglitz also played a significant role in promoting and exhibiting the work of emerging American artists, including the renowned artist Georgia O'Keeffe, who would later become his wife.
Stieglitz's galleries, particularly '291', were influential in introducing avant-garde European artists to the American art scene, significantly shaping the development of modern art in the United States.