Sound waves in air are always longitudinal because:
A. of the inherent characteristics of sound waves in air
B. air does not have a modulus of rigidity
C. air is a mixture of several gases.
D. density of air is very small

Hint: Longitudinal waves are the type of mechanical waves in which particles of the medium vibrate along the direction of propagation of waves in the form of compressions and rarefactions. Longitudinal waves propagate through elastic medium only and elastic medium have no modulus of rigidity.


Complete step-by-step answer:
Longitudinal waves are the type of mechanical waves in which particles of the medium vibrate along the direction of propagation of waves in the form of compressions and rarefactions.
Sound is the acoustic wave that can propagate through gases, liquids and solids. These waves only travel through an elastic medium. Thus, if the medium is rigid then waves will not travel through that medium.
Since sound travels in the form of compression and rarefactions, particles of the medium should oscillate back and forth along the direction of propagation of the wave. For the oscillations of the particles medium should have minimum modulus of rigidity.
Air is medium having no modulus of rigidity. Hence sound waves travelling through the air are always longitudinal.
Indeed it is true that air is a mixture of several gases and its density less but this is not the appropriate reason for sound waves being longitudinal in air medium.
Sound is the acoustic wave that can propagate through gases, liquids and solids. These waves only travel through an elastic medium. Thus, if the medium is rigid then waves will not travel through that medium.
Since sound travels in the form of compression and rarefactions, particles of the medium should oscillate back and forth along the direction of propagation of the wave. For the oscillations of the particles medium should have minimum modulus of rigidity.
Air is medium having no modulus of rigidity. Hence sound waves travelling through the air are always longitudinal.
Indeed it is true that air is a mixture of several gases and its density less but this is not the appropriate reason for sound waves being longitudinal in air medium.
Additional information:

There are two types of mechanical waves: Longitudinal waves and transverse waves. In transverse waves, particles of the medium vibrate perpendicular to direction of propagation of the wave. Light waves are the transverse waves.

Note: It is a common misconception that sound waves are always longitudinal waves. Sound can travel in the form of longitudinal waves through gases, liquids and solids. Sometimes it travels in the form of transverse waves in solids. Sound waves are generated by a vibrating source. The greater is the density of the medium, greater will be the speed of sound.