1. Key idea: A wave is called transverse if its vibrations are perpendicular to the direction of propagation.

2. We need to identify which phenomenon specifically proves that electromagnetic waves are transverse.

3. Let us examine the options:

   • A: Polarization
     Polarization is a property shown only by transverse waves. In polarization, the vibrations of the wave are restricted to a particular plane. Since longitudinal waves cannot be polarized, the observation of polarization in electromagnetic waves proves that they are transverse.

   • B: Interference
     Interference is shown by both transverse and longitudinal waves, so it does not prove transverse nature.

   • C: Reflection
     Reflection can occur for both transverse and longitudinal waves, so it does not establish transverse nature.

   • D: Diffraction
     Diffraction is also shown by both types of waves, so it does not prove transverse nature.

4. Therefore, the phenomenon that evidences the transverse nature of electromagnetic waves is:

$\boxed{\text{Polarization}}$

So, the correct option is A.