Answer: The correct answer is natural frequency.
Step-by-step explanation:
Resonance occurs when the applied frequency of the object is equal to its natural frequency.
For example, the soldiers are told not to march on the bridge because the applied frequency on the bridge is equal to the natural frequency of the bridge. The bridge will collapse as resonance occurs in this case.
Therefore, for resonance to occur, the object must experience a frequency equal to its natural frequency.