Explanation: At resonance, the inductive reactance ($X_L$) and capacitive reactance ($X_C$) in a circuit are equal in magnitude and opposite in their phase effect. This mutual cancellation means the net reactive impedance of the circuit becomes zero. Consequently, a resonant circuit behaves as if it were purely resistive, with the total impedance ($Z$) being equal to the circuit's resistance ($R$). In any purely resistive circuit, the voltage across the component and the current through it are always in phase. This means they reach their peak values, zero crossings, and minimum values at precisely the same time. Therefore, in a resonant circuit, the voltage and current are in phase. Options B and D describe a 90-degree phase shift, which occurs in purely inductive or purely capacitive circuits (current lags voltage in inductors, leads in capacitors). Option A, a 180-degree phase shift, is not characteristic of the voltage-current relationship in a basic resonant RLC circuit.