The correct option is D forced oscillations
For resonance to occur the frequency of external periodic force should be equal to natural frequency of oscillation of the system or it should be very close to the natural frequency.
where f= frequency of external periodic force
f0=Natural frequency of oscillation for system
The response of system towards the periodic force is considered at steady state condition, when disturbances get stabilized with time.
⇒Hence resonance is a special case of forced vibrations, when frequency of the driving force is equal to the natural frequency of the body. Resonance is the result of an external force vibrating at the same frequency as the natural frequency of a system.