Q.Which of the following phenomenon proves that light is a transverse wave? (A) reflection
(B) interference
(C) diffraction
(D) polarization

[!TLDR] Only a TRANSVERSE wave can have its oscillation direction restricted to one plane, which is exactly what a polarizer does -- reflection, interference and diffraction all occur for longitudinal waves too, so none of them alone proves transverse-ness. [!ANSWER] (D) polarization

Reflection and refraction are exhibited by both transverse and longitudinal waves (e.g. sound, a longitudinal wave, reflects and refracts perfectly well), so observing them says nothing about which type of wave light is. Interference and diffraction likewise occur for longitudinal waves -- sound interferes and diffracts too -- so these also fail to distinguish transverse from longitudinal. Polarization, however, is fundamentally a statement about the DIRECTION of oscillation being restricted to a single plane perpendicular to the propagation direction; this operation is only even MEANINGFUL for a transverse wave, since a longitudinal wave's oscillation is always along the direction of propagation itself, with no perpendicular direction to restrict in the first place (Section 7.7.2's 'Remember this' point: longitudinal waves like sound cannot be polarized at all). Since light CAN be polarized (Section 7.7, Malus' Law, Brewster's law all rely on it), this is direct experimental proof that light's oscillation lies perpendicular to its direction of travel -- i.e. that light is a transverse wave. [!ANSWER] (D) polarization

Recall that polarization -- restricting a wave's oscillation direction to one plane -- is only physically meaningful for a transverse wave; a longitudinal wave has no perpendicular oscillation direction to restrict, so only transverse waves can be polarized at all.

Picking interference or diffraction because they were the other 'exotic' wave phenomena covered in the chapter, without checking that these are exhibited by longitudinal waves (like sound) too, so they cannot specifically prove transverse-ness.