Sound waves in air are ______ waves.

Sound waves in air are longitudinal waves. In longitudinal waves, the displacement of the medium's particles occurs parallel to the direction of wave propagation.

As a sound wave travels through air, it creates regions of compression (where air molecules are pushed together) and rarefaction (where air molecules are spread apart). This alternating pattern of compressions and rarefactions allows the sound energy to move through the medium.

In contrast, transverse waves involve particle displacement perpendicular to the direction of wave propagation, which is characteristic of electromagnetic waves like light. Therefore, options b (transverse), c (radio), and d (electro-magnetic) do not accurately describe sound waves in air.

It's important to note that while sound waves in fluids (gases and liquids) are longitudinal, in solids, sound can propagate as both longitudinal and transverse waves.

Therefore, the correct answer is a. longitudinal.