291 was the New York gallery that first exhibited Georgia O'Keeffe's drawings in 1916.
The gallery's formal address was 291 Fifth Avenue, and it became known by that number. Alfred Stieglitz founded it in 1905 as the Little Galleries of the Photo-Secession. It later became a crucial venue for photography and modern art in the United States.
Stieglitz first encountered O'Keeffe's abstract charcoal drawings through his friend and fellow artist Anita Pollitzer. Without O'Keeffe's knowledge, Pollitzer showed the drawings to Stieglitz, who exhibited them at 291 in 1916. That exhibition brought O'Keeffe into New York's avant-garde art network.
The gallery closed in 1917, but its influence continued through Stieglitz's later exhibition spaces. The Intimate Gallery and An American Place were separate Stieglitz venues, which is why they are plausible but incorrect answers to this question.