The frequency of a driven oscillator (the glass) is matched to that of the driving force (the sound waves emanating from the singer).
To get this shattering effect, the amplitude of vibration of the glass must be large; this suggests that the natural vibration frequency of the glass is similar to the frequency of the note being sung (so as to give resonance).
[There is ONE correct answer above. Choose the one you think is correct and press the "Check Your Answer" button below to see if you are right.]