What happens to the phase relationship of current and voltage in a resonant circuit?

In a resonant circuit, the key characteristic is that the impedance is at a minimum, and the circuit can efficiently transfer energy between the inductive and capacitive components. At resonance, the inductive reactance (which causes the current to lag the voltage) and the capacitive reactance (which causes the current to lead the voltage) are equal in magnitude but opposite in effect. As a result, these opposing effects cancel each other out, and the current and voltage become in phase with each other.

This in-phase condition means that at resonance, the peaks and troughs of the voltage waveform and the current waveform occur at the same time, maximizing the power transfer in the circuit. The impedance at resonance is purely resistive, which contributes to this phase relationship.

Other options suggest different scenarios that don't accurately apply to resonant circuits. For instance, saying they always remain out of phase is not true at resonance, where they align. Stating that they become irrelevant misconceives the importance of the phase relationship in determining circuit behavior. The option that suggests only voltage leads misrepresents the interactions between current and voltage in a resonant circuit where both become in phase instead.