Longitudinal waves are a type of mechanical waves where the vibrations of the particles are parallel to the direction of wave propagation. These waves can undergo reflection, diffraction, and superposition with other waves. However, longitudinal waves cannot be polarized, which means their vibrations cannot be restricted to a single plane. This is because the particles in the wave are oscillating back and forth in the same direction as the wave is traveling, so there is no way to restrict their motion to a single plane. Therefore, the answer is polarized.