An opera singer breaks a crystal glass with her voice. which best explains why this occurs?

Any physical object has frequencies at which they naturally vibrate, known as resonance frequencies. If you flick a crystal wine glass with your finger, you will hear a fairly clear tone as the glass vibrates, causing waves of air pressure to emanate out from it, which your ears and brain interpret as sound. The sound gradually gets quitter and dies out as the amplitude of the vibrations diminishes due to energy being carried away by the sound waves. 

Your voice is also a series of air pressure waves, with the pitch related to the frequency of the waves, and the volume related to the amplitude of the waves. 

Now to fit these two together. If you can match the pitch of your voice to the resonant frequency of the glass the vibrating air will start the glass vibrating too. If you can do this with sufficient volume, the glass will try to move in its vibration farther and faster than the material in the glass is able to move, and the glass will break under the strain. 

This is an example of a driven oscillation. Imagine pushing a friend on a swing: after one big push, the swing slows down, but continues oscillating for a while with a given frequency. If you impart randomly timed pushes to your friend, you are unlikely to get her moving very much. But if you carefully time your efforts so you administer a small push at the same time in each cycle, your efforts can add up and your friends amplitude (height) will increase a bit each time. Likewise, the sound of your voice administers hundreds of tiny 'pushes' each second to everything around you. If the timing is right, the energy of the pushes can add up, leading to a large stress on the glass.

Some glass can wistand more ( resonant frequency ) but breaking class has to do with hertz. some break at 550