Meritnation experts i did not understand the ans clearly plz explain me again

Dear Student, 

Velocity = Displacement/time

Now, displacement is the change in position of object. Distance is the magnitude of distance covered by an object.
Distance equals to displacement when motion is unidirectional. 
When direction of motion changes, distance is not equal to displacement.

For a wave, the motion is unidirectional. So, displacement in case of wave is equal to distance covered by it.

Hence, velocity can be given as distance/time.

Regards