The correct answer is: A. unity.
Voltage regulation is the change in output voltage of a power supply or electrical transmission line, expressed as a percentage of the no-load voltage, when the load is changed.
When the power factor of the load is unity, the load is resistive and there is no reactive power being drawn from the source. This means that the current and voltage are in phase, and the only power loss is due to resistance.
When the power factor of the load is lagging, the load is inductive and there is reactive power being drawn from the source. This means that the current is lagging the voltage, and the power loss is due to both resistance and inductance.
When the power factor of the load is leading, the load is capacitive and there is reactive power being supplied to the source. This means that the current is leading the voltage, and the power loss is due to both resistance and capacitance.
In all cases, the minimum voltage regulation occurs when the power factor is unity. This is because there is no reactive power being drawn from the source, and the only power loss is due to resistance.