When the frequency of the driving force equals the natural frequency of the system, the system is said to be in. (a) Resonance (b) Equilibrium (c) Elastic limit (d) None

Option A is the correct answer.

Explanation:

When the frequency of the driving force equals the natural frequency of the system, the system is said to be in resonance. At resonance the system vibrates in maximum amplitude.

Marching soldiers are cautioned to break stride on a bridge is because of resonance, if the frequency of soldiers stride is equal to frequency of bridge, the bridge will vibrate with maximum amplitude. This will in turn collapse bridge.

Option A is the correct answer.