Ab observer detects an apparent change in the frequency of sound waves produced by an airplane passing overhead. this phenomenon illustrates a. the doppler effect b wave amplitude increase c the refraction of sound waves d wave intensity increase

Hello There

The Doppler effect

Reason: The Doppler effect is an increase or decrease in the frequency of sound, light,  or waves as the source and observer move to each other. An example would be a firetruck with its siren on driving away from you.

I hope this helps
-Chris