The Hirshhorn Museum and Sculpture Garden was added to the Smithsonian in the 20th century, specifically in 1966. That year, collector Joseph H. Hirshhorn agreed to give his major art collection to the nation, and Congress passed legislation accepting the gift and assigning the Smithsonian responsibility for a new museum on the National Mall.
The museum was created to house Hirshhorn’s collection of modern and contemporary art. Its distinctive building was designed by architect Gordon Bunshaft as an elevated, cylindrical structure surrounded by sculpture grounds. Construction began in 1969, and the museum opened to the public in October 1974.
A common mix-up is to answer 1974 because that is when the museum opened. However, the question asks when it was added to the Smithsonian, which refers to its establishment through the 1966 congressional act. The Hirshhorn became the Smithsonian’s modern-art museum, while its Sculpture Garden provided an outdoor setting for large-scale works on the Mall.