G.P. Thomson experimentally confirmed the existence of matter waves by the phenomena:
A. Diffraction
B. Refraction
C. Polarization
D. Scattering

Hint: To get the answer for this question, first understand the details of the experiment conducted by G.P. Thomson and what the main reason was to conduct the experiment and what was the output of this experiment. Once all the details are gathered, it will help us to know what type of phenomena was confirmed.

Complete solution:
G.P. Thomson had experimentally confirmed the existence of matter waves, which was stated as de Broglie's hypothesis, in the experiment it demonstrated that electron beams are diffracted when they are scattered by the regular atomic rays of crystals.

From this experiment it was found that electrons get scattered from atoms in a crystal and these scattered electrons produced an interference pattern. These diffraction patterns are characteristic of wave-like behavior and are exhibited by both matter and electromagnetic radiation. Diffraction patterns are obtained if the wavelength is comparable to the spacing between scattering centers.

Hence, the correct option is A.

Additional Information:
Matter waves are a central part of the theory of quantum mechanics, being an example of wave–particle duality. All matter exhibits wave-like behavior. For example, a beam of electrons can be diffracted just like a beam of light or a water wave. In most cases, however, the wavelength is too small to have a practical impact on day-to-day activities. Hence in our day-to-day lives with objects the size of tennis balls and people, matter waves are not relevant.
The concept that matter behaves like a wave was proposed by Louis de Broglie in 1924. It is also referred to as the de Broglie hypothesis. Matter waves are referred to as de Broglie waves.

Note:
It is necessary to understand the details related to the experiment to get the answer for this solution. There are some other experiments as well, that is important to remember, as there are direct questions related to those experiments that are asked in the exams. To answer all the questions related to experiments, it is necessary to have a thorough understanding of these experiments.