The is the frequency at which a material tends to vibrate when it is disturbed.

correct fill up is natural frequency

Explanation:

solution

Vibration is a mechanical phenomenon and vibrating any object it will create the disturbance, it may be vocal cord of a person and any object that is vibrating, it will create sound and Decibel dB is unit which is use to measure the intensity of sound

All the object having natural frequency at which they vibrate  

and frequency at which object tends to vibrate with when hit or plucked or struck or if some how it disturb, that is natural frequency

so correct fill up is natural frequency

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answer; (d) one tectonic plate plunges beneath another; (most common,convergent boundaries)