From the record
“The Sitting Bull Monument, on Standing Rock Sioux Reservation near Mobridge in Corson County, South Dakota, was built in 1953. It was listed on the National Register of Historic Places in 2006.”
Wikipedia article Sitting Bull Monument, CC BY-SA 4.0.
One of 4 National Register listings published here for Corson County · listed in 2006 · category: Object · built in 1953.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service