The correct answer is D. both (B) and (C).
The efficiency of a transformer is the ratio of the output power to the input power. It is a measure of how well the transformer converts electrical energy from one form to another. The efficiency of a transformer is affected by a number of factors, including the supply frequency, the load current, and the power factor of the load.
The supply frequency is the rate at which the voltage and current in the transformer’s primary winding change. The higher the supply frequency, the higher the efficiency of the transformer. This is because the higher frequency causes the magnetic flux in the transformer to be more concentrated, which reduces the losses in the transformer.
The load current is the current that flows through the transformer’s secondary winding. The higher the load current, the lower the efficiency of the transformer. This is because the higher load current causes the transformer to heat up, which increases the losses in the transformer.
The power factor of the load 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 of a load is usually less than 1, which means that the load is not using all of the power that is being supplied to it. The lower the power factor of the load, the lower the efficiency of the transformer. This is because the transformer has to supply more current to a load with a lower power factor, which causes the transformer to heat up and increases the losses in the transformer.
In conclusion, the efficiency of a transformer depends on the supply frequency, the load current, and the power factor of the load. The higher the supply frequency, the higher the load current, and the higher the power factor of the load, the higher the efficiency of the transformer.