Final answer:
Arthur Houghton donated parts of his art collection to The Metropolitan Museum of Art, Reverend Freer donated to the British Museum, and after Alfred Stieglitz's death, Georgia O'Keeffe donated his photographs to the National Gallery of Art.
Step-by-step explanation:
The question, To whom did he donate his art collection?, refers to several individuals throughout history who have donated art collections. Specifically, Arthur Houghton donated 76 illustrated folios from the Tahmasp Shahnama to The Metropolitan Museum of Art in 1970. Additionally, Reverend Freer passed his collection on to the British Museum in 1900. Lastly, after Alfred Stieglitz's death, Georgia O'Keeffe provided over 1,600 of his photographs to the National Gallery of Art. In these instances, substantial art donations were made to prominent institutions, enhancing public access to culturally significant works.