What device measures the actual displacement of the ground with respect to a stationary reference point?

The device that accurately measures the actual displacement of the ground with respect to a stationary reference point is the displacement meter. Displacement meters are specifically designed to detect and record any changes in position or shifts in the earth's surface, providing essential data for monitoring subsidence, landslides, or other ground movements.

While a seismograph is involved in measuring ground motion, it primarily records the vibrations caused by seismic activities. An inclinometer is used to measure the angle of tilt or slope rather than direct displacement, and an accelerometer is designed to measure acceleration forces, not the actual displacement itself. Thus, the displacement meter stands out as the most direct means of assessing ground movement relative to a fixed point, making it the correct choice.