Explanation :
When the frequency at which the applied force is equal to the natural frequency of the object, then resonance occurs.
Natural frequency means the frequency at which the system tends to oscillate without any external force.
The natural frequency for a spring- mass system is given by:
[tex]\omega_0=\sqrt{\dfrac{k}{m}}[/tex]
where k is the spring constant
and m is the mass of spring.
Hence, resonance is the right answer.
