Step 1: Understanding vibration behavior.
In vibration systems, amplitude may remain constant or decrease depending on energy loss mechanisms such as friction or damping.
Step 2: Effect of damping.
When damping is present, energy is dissipated in every cycle, causing the amplitude to reduce progressively with time.
Step 3: Eliminating incorrect options.
Free vibration occurs without external force but may or may not include damping.
Forced vibration occurs due to continuous external excitation.
Underdamped vibration is a specific case of damped vibration with oscillations present.
Step 4: Conclusion.
A reduction in amplitude over each cycle indicates damped vibration.