Answer:
Sound waves are longitudinal in nature.
Step-by-step explanation:
There are many types of waves like transverse, longitudinal, electromagnetic wave etc.
Sound waves are longitudinal in nature. In longitudinal type of wave, the medium particles moves parallel to the propagation of the wave. This type of waves move in the form of compression and rarefaction.
In compression, the particle density at a point is very less while in rarefaction, the particle density at a point is very high.
So, the correct option is (b) "longitudinal wave".