Final answer:
The term for an aircraft becoming invisible to radar due to a Doppler shift matching the radar's PRF is 'Doppler Cloaking' (option C). This effect is due to the radar's inability to distinguish the aircraft from background signals when the frequency of the returning waves is altered by the aircraft's motion.
Step-by-step explanation:
The term that describes the condition of an aircraft becoming invisible to radar when it travels at a certain speed and creates a Doppler shift that matches the pulse repetition frequency (PRF) of the radar is Doppler Cloaking (option C).
Radar devices emit microwaves and analyze the echoes that reflect off objects to determine their location and speed. When an object, like an aircraft, moves towards or away from the radar, the frequency of the reflected waves shifts—this is known as the Doppler effect. If the reflected waves undergoing the Doppler shift have a frequency that coincides with the radar's PRF, the returned signal can blend with the background noise, making the aircraft effectively invisible to that particular radar system. This technique does not render the aircraft completely 'stealth,' but rather exploits a specific vulnerability in the radar's detection capability.