What is Doppler shift in reference to satellite communications?

Answer: C. An observed change in signal frequency caused by relative motion between the satellite and Earth station

Doppler shift is an observed change in signal frequency caused by relative motion between the satellite and Earth station. As the satellite comes toward you, its frequency appears higher; as it moves away, lower. It is like a passing car's siren changing pitch.

Tip: Approaching = higher, leaving = lower.