It appears what appears to exist make waves move further into the class a b c waves produced​

In physics, mathematics, and related fields, waves are dynamic disturbances that propagate (change from equilibrium) of one or more quantities, sometimes as similar to the wave equation. In physical waves, there are at least two field magnitudes in the medium wave involved. Waves can be periodic, in this case the quantity oscillates repeatedly at an equilibrium (resting) value at a certain frequency. When the entire waveform moves in one direction, it is called a traveling wave; Rather, overlapping periodic wave pairs moving in opposite directions create standing waves. In standing waves, the amplitude of the vibration has zero in some positions where the amplitude of the wave appears to be smaller or even zero.

Sorry if wrong

the answer is   as follows

d

c

b

a

d