Which of the following waves cannot be polarised?

Only transverse waves (such as radio, ultraviolet and infrared waves) can be polarized since 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. However, sound waves are longitudinal in which vibrations are parallel to the direction of travel of the wave. The vibrations of a longitudinal wave occur along a single line, it is therefore not possible to confine that to a plane. So it cannot be polarized.