Step 1: Understanding reference surfaces for elevation.
- Geoid is the equipotential surface of Earth's gravity field, representing the mean sea level used as a reference for elevation measurements.
- Datum is a coordinate reference system based on a geoid or ellipsoid.
- Ellipsoid is a mathematical approximation of Earth's shape.
- Sphere is an oversimplified model of Earth, rarely used in geodesy.
Step 2: Selecting the correct option.
Since the geoid is the actual reference for mean sea level elevation, the correct answer is c. Geoid.