Explanation: Power factor is a crucial measure in AC circuits, indicating how efficiently electrical power is being utilized. Option A correctly states that power factor is the ratio of resistance (R) to impedance (Z). In an AC circuit, impedance is the total opposition to current flow, combining both resistive and reactive components. Resistance dissipates energy as useful power, while reactance (from inductors and capacitors) stores and returns energy to the source without performing useful work. This ratio (R/Z) quantifies the portion of the total impedance that contributes to actual power consumption. Option B also correctly defines power factor as the ratio of true power (P, measured in watts) to apparent power (S, measured in volt-amperes). True power is the actual power consumed by resistive loads that performs useful work. Apparent power is the total power supplied by the source. This ratio (P/S) represents the efficiency of power delivery. Both definitions are mathematically equivalent and arise from the phase relationship between voltage and current in an AC circuit. Therefore, both A and B are valid descriptions of power factor.