Explanation: Resonant frequency occurs when the inductive reactance ($X_L$) of a circuit precisely equals its capacitive reactance ($X_C$). At this specific frequency, these opposing reactances cancel each other out. This makes the circuit appear purely resistive, allowing maximum current flow in a series RLC circuit or maximum voltage across the reactive components in a parallel RLC circuit. This principle is fundamental for tuning radio receivers and transmitters, allowing selection of a specific frequency. Options B and C are incorrect because resonant frequency is a specific condition where reactances balance, not simply the highest or lowest frequency that will pass current, which relates more to filter characteristics. Option D is also incorrect; at resonance, the power factor is actually at its *maximum* (unity, or 1), indicating that all power is real and consumed by resistance, with no reactive power.