Which phenomenon best supports the theory that matter has a wave nature?

At the beginning of the 20th century (especifically 1924) the French physicist Louis De Broglie proposed in its doctoral thesis the existence of matter waves, that is to say that all matter has a wave associated with it.

De Broglie, in addition, deduced an equation by which the electron had a wavelength that depended on its momentum (hence its velocity).

These postulations were tested with the double slit experiment (formerly applied to photons) applied to electrons, and the result was: electrons (as well as the other particles different from the photons) are able to behave as waves.

It is important to note, this experimente was done in 1927 by Clinton J. Davisson and Lester Halbert Germer and was called the electron diffraction experiment. It consisted of bombarding with an electron beam a sample (nickel) and observing the resulting interference pattern.

In this way they demonstrated what de Broglie deduced mathematically.

a.) travels faster in solids because the particles are closer together.

air is a gas water is a liquid