B

Step 1: Understand the basic nature of sound waves. Sound waves are mechanical waves that propagate through a medium by displacing particles of that medium.
Step 2: Identify the types of mechanical waves:

• Transverse Waves: Particles of the medium move perpendicular to the direction of wave propagation (e.g., waves on a string).
• Longitudinal Waves: Particles of the medium move parallel to the direction of wave propagation (e.g., sound waves in gases).

Step 3: In gases, sound propagates as a series of compression and rarefaction, which directly indicates that the particle displacement is parallel to the direction of energy transport, classifying it as a longitudinal wave.
Conclusion: Therefore, the nature of sound waves in gases is longitudinal.