Which spheroid is used for the NAD83 datum?

The North American Datum of 1983 (NAD83) utilizes the Geodetic Reference System 1980 (GRS80) spheroid. This spheroid was specifically designed to provide a more accurate representation of the Earth's shape for North America and is based on measurements of the Earth's gravity field. GRS80 is compliant with modern geodetic technologies and was established to improve upon earlier models, offering a more precise reference for mapping and navigation.

The GRS80 spheroid takes into account the Earth's flattening and has parameters that reflect a global standard as opposed to regionally focused models like the Clarke 1866 spheroid. While Clarke 1866 is used in older datums like NAD27, NAD83 represents a significant evolution in geodetic reference systems, necessitating the adoption of GRS80.

In contrast, the other spheroids listed are either associated with different datums or were used prior to the establishment of NAD83. For modern geospatial applications that require high accuracy in North America, the GRS80 spheroid is the appropriate choice, making it the correct answer.