You're welcome. SD Historical Society site says that these are buried in Evergreen Cemetery, Hot Springs, Fall River Co., SD.
DRENKHAHN, George, d. 26 Nov. 1926; & Naida Ruth, d. 20 May 1922.
Death certificates should be available, and those are supposed to show the names of the parents of the deceased.
In South Dakota, the Register of Deeds of the county should be able to tell you who is in charge of the cemetery.