Which of the following defines resonance?

Resonance is a phenomenon that occurs when a system is driven by an external force at a frequency that matches one of its natural frequencies of oscillation. In the context of magnetic resonance, particularly MRI, resonance refers to the condition in which the magnetic moments of the nuclei in a magnetic field absorb energy from a radiofrequency (RF) pulse. When the frequency of the RF pulse is equal to the natural frequency of the magnetic moments, they gain energy, resulting in the absorption that contributes to signal generation in MRI.

This principle underlies the effectiveness of MRI, where the absorption of RF energy leads to changes in the state of the atomic nuclei, allowing the imaging process to occur. Therefore, energy gain at a matching frequency accurately encapsulates the essence of resonance in this context.