Which datum was the first calculated for the United States?

The National Spatial Reference System (NSRS) is underpinned by various datums, and NAD27, or the North American Datum of 1927, is significant as the first datum specifically calculated for the United States. This datum was developed to provide a common reference frame for geodetic surveys and mapping across North America.

NAD27 was based on the earlier geodetic surveys conducted to determine precise locations across the continent, utilizing points from several established benchmarks. It helped in standardizing mapping efforts and improving the accuracy of geographic data during that era.

The other options represent different datums with varying focuses and purposes. For instance, NAD83 is an advancement over NAD27, employing satellite technology for greater accuracy in the later 20th century; WGS84 is a global datum developed for use in GPS and is widely adopted internationally; NAD39 was an interim system before the adoption of NAD83 and is less recognized.

In summary, NAD27's historical role as the first datum calculated for the United States is what makes it the correct choice in this context.