If a wave can be polarized, it means, it is:

Explanation

Only transverse waves can be polarised. Longitudinal waves cannot be polarised.Transverse: vibrations perpendicular to the direction of travel of the wave.Longitudinal: vibrations parallel to the direction of travel of the wave.Sound waves are longitudinal so they cannot be polarised.Polarised: the wave vibrations occur in one plane onlyUnpolarised: the wave vibrations occur in all planes perpendicular to the direction of travel.The reason that only transverse waves can be polarised is that their vibrations can potentially occur in all directions perpendicular to the direction of travel. It is therefore possible to confine the vibrations to a single plane. Or put another way an unpolarised transverse wave is a 3D shape, it is therefore possible to confine it to a 2D shape (a plane). The vibrations of a longitudinal wave occur along a single line, it is therefore not possible to confine that to a plane, i.e. a 1D shape cannot be confined within a 2D shape. Similarly, Stationary waves and superimposed waves can not be polarised as they are along a single line.