Answer:
A) Doppler effect
Step-by-step explanation:
As we know that when Sound source travel with respect to observer then due to relative motion of source and observer the frequency of sound emitted by the source is observed slightly different by the observer.
This change in the frequency of the sound observed by the observer is known as Doppler's Effect
So here we know by the formula of doppler's effect the frequency is given as

so here since the frequency observed is different from real frequency so the wavelength observed by the observer is changed
so correct answer will be
A) Doppler effect