Answer : Particles vibrate parallel to the direction the sound travels.
Explanation :
Sound is a longitudinal wave. The particles of medium moves in the direction of the propagation of the wave. The sound waves requires a medium to travel.
The longitudinal wave propagates in the form of compression and rarefaction. The distance between the compression and the rarefaction is called wavelength.
So, the correct option is (d) " Particles vibrate parallel to the direction the sound travels".
