RADAR stands for RAdio Detection And Ranging. Its operation is based entirely on the reflection of electromagnetic waves, which is why that is the correct answer.

The basic sequence of operation is:

1. A transmitter generates a short, high-power pulse of electromagnetic energy — usually in the microwave/radio band — which is radiated into space by a directional antenna.
2. When the pulse strikes an object such as an aircraft, ship, or vehicle, part of the energy is scattered and reflected back toward the radar. This returned energy is called the echo.
3. The same (or a nearby) antenna captures the weak echo and passes it to a sensitive receiver.
4. Signal processing extracts the target information: the round-trip time delay gives the range (distance = c × t / 2, where c is the speed of light), the Doppler frequency shift of the echo gives the target's radial velocity, and the antenna pointing angle gives its direction.

Because radar depends on electromagnetic reflection, it works even in darkness, fog, or cloud, where optical sensing would fail.

Acoustic resonance is incorrect — sound-based detection is used in SONAR, which works underwater where sound propagates well but radio waves are heavily attenuated. Atmospheric refraction does bend radar waves slightly and can extend or distort coverage, but it is a propagation side-effect, not the mechanism that reveals a target's presence. Chemical ionization has nothing to do with detecting a reflecting object; it relates to processes such as gas-discharge and mass spectrometry.