Question

an opera singer breaks a crystal glass with her voice. which best explains why this occurs?
the pitch of her voice matches the pitch of the glass.
the amplitude of her voice heats the glass until it breaks.
the intensity of her voice produces a standing wave in the glass.
the frequency of her voice matches the natural frequency of the glass.

Explanation:

When the frequency of the opera singer's voice matches the natural frequency of the glass, resonance occurs. Resonance causes the glass to vibrate with increasing amplitude until it breaks. Pitch is related to frequency but the key concept here is the match of the frequency of the voice (external force) with the natural frequency of the glass. Amplitude alone (without frequency match) won't cause the glass to break in this context. Intensity is related to energy (related to amplitude squared) but again without frequency - natural frequency match, it won't lead to the glass breaking. Standing waves can be part of resonance phenomenon but the core reason is the frequency - natural frequency match.

Answer:

The frequency of her voice matches the natural frequency of the glass.