Polarization distinguishes transverse from longitudinal waves

Steps:

• Transverse waves have particle motion perpendicular to propagation direction; longitudinal waves have motion parallel to it.
• Evaluate option A: diffraction occurs in both wave types due to wave nature.
• Evaluate option B: only transverse electromagnetic waves propagate in vacuum; longitudinal waves require a medium.
• Evaluate option D: polarization restricts oscillation direction, possible only in transverse waves.

Why D is correct:

• Polarization aligns wave oscillations in a specific plane perpendicular to propagation, a property exclusive to transverse waves by definition.

Why the others are wrong:

• A: Both transverse and longitudinal waves diffract when passing obstacles comparable to wavelength.
• B: Longitudinal waves, like sound, cannot travel in free space without a medium; transverse waves, like light, can.
• C: Coherence depends on phase relationship, achievable in both wave types (e.g., coherent sound waves exist).

Final answer: D