In a medium, the nature of sound waves is:

Concept: A wave is classified by how its particles vibrate relative to the direction the wave travels. In a longitudinal wave, particles vibrate back and forth parallel to the direction of propagation, creating a moving pattern of compressions (particles close together) and rarefactions (particles spread apart). In a transverse wave, particles vibrate perpendicular to the direction of propagation, creating crests and troughs; this requires the medium to resist sideways (shear) displacement.

Application: A vibrating sound source pushes and pulls the particles of the medium along the very same line the sound travels, building a moving sequence of compressions and rarefactions. This back-and-forth motion parallel to the direction of travel is exactly the definition of a longitudinal wave, so sound waves in a medium are classified as longitudinal.

Cross-check: Testing the transverse alternative confirms this: a transverse wave needs the medium to resist a sideways (shear) displacement of its particles, a property fluids such as air and water do not have since they flow freely under shear. Because sound travels through such media as a compression wave, it cannot be transverse, it is not ‘neither’ (it clearly does have a definite classification), and it is not ‘both’ (a wave's classification follows from how the medium restores displaced particles, not an arbitrary combination). So the correct classification is only longitudinal.

Note: Solids are a special exception: because they can resist shear, they may additionally carry a transverse (shear) wave component alongside the longitudinal one. The classification tested here is the standard one taught for sound in a general medium, per the exam's own answer key.