The correct answer is A. Ratio of net reactance and impedance.
The power factor of a series R-L-C circuit is defined as the ratio of real power to apparent power. Real power is the power that is actually used by the load, while apparent power is the total power that is delivered to the load. The power factor is a dimensionless number between 0 and 1, and it is usually expressed as a percentage.
The net reactance of a series R-L-C circuit is the difference between the inductive reactance and the capacitive reactance. The impedance of a series R-L-C circuit is the square root of the sum of the squares of the resistance, inductive reactance, and capacitive reactance.
The ratio of net reactance and impedance is not a correct definition of power factor because it does not take into account the resistance of the circuit. The power factor is a measure of how efficiently the power is being used by the load, and it is affected by both the reactance and the resistance of the circuit.
The other options are all correct definitions of power factor. Option B is the ratio of kW to kVA, which is the same as the ratio of real power to apparent power. Option C is the ratio of J to Z, which is the same as the ratio of reactive power to apparent power. Option D is the ratio of W to VA, which is the same as the ratio of real power to total power.