A stationary RADAR instrument captures the true speed of an object only when:

The correct choice indicates that a stationary RADAR instrument accurately captures the true speed of an object only when that object is moving directly away from or toward the RADAR. This is due to the principles of Doppler radar technology, which measures the change in frequency of the returned signal to determine speed. When an object moves directly towards or away from the RADAR unit, the Doppler effect produces a clear frequency shift that correlates directly with its true speed.

In contrast, if the object is stationary, the radar would measure no change in speed since there is no movement to capture. When the RADAR is in motion, it can introduce complexities in measuring speed as the relative motion may affect the readings. Lastly, if the object is moving at an angle, the measured speed will not reflect the true speed of the object due to components of movement not being directly along the line of sight. Thus, only direct movement toward or away from the RADAR results in accurate speed measurement.