The correct answer is D. unity power factor.
A transformer is a device that transfers electrical energy from one circuit to another without changing the frequency of the power. It does this by using two coils of wire that are magnetically coupled. The primary coil is connected to the source of power, and the secondary coil is connected to the load.
The power factor of a transformer is the ratio of the real power (the power that is actually used by the load) to the apparent power (the product of the voltage and current). A power factor of unity means that all of the power that is supplied to the transformer is used by the load. A power factor of less than unity means that some of the power is being wasted as heat.
The efficiency of a transformer is the ratio of the output power to the input power. The efficiency of a transformer is affected by the power factor. A transformer with a power factor of unity will be more efficient than a transformer with a power factor of less than unity.
Therefore, for a transformer, operating at constant load current, maximum efficiency will occur at unity power factor.