The correct answer is C. Resistance.
The power factor of a circuit is the ratio of the real power to the apparent power. The real power is the power that is actually used by the load, while the apparent power is the product of the voltage and current. The power factor is equal to one when the load is purely resistive, and it decreases as the load becomes more inductive or capacitive.
Inductance and capacitance are both reactive loads, which means that they store energy rather than dissipate it. This energy storage causes the current to lag behind the voltage, which reduces the power factor. Resistance, on the other hand, is a resistive load, which means that it dissipates energy rather than storing it. This causes the current to be in phase with the voltage, which results in a power factor of one.
Therefore, the power factor of a circuit will be unity when the load is purely resistive.