The most secure choice for a device that utilizes electromagnetic waves to determine the position and velocity of an object would generally be radio waves.
Radio waves are commonly used in radar and various other positioning systems, such as GPS, because they are less affected by obstacles and environmental conditions compared to higher frequency waves like X-rays or gamma rays. Additionally, they are more secure in terms of privacy and safety, as they do not have the ionizing properties of X-rays and gamma rays that could have harmful biological effects.
Therefore, while the option is incomplete, if you're considering radio waves as a choice, they would typically be the most appropriate for such applications due to their practicality and safety.