The correct answer is A. low power factor wattmeter.
A wattmeter is an instrument used to measure the power in an electric circuit. It does this by measuring the voltage and current in the circuit and multiplying them together. 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.
A low power factor wattmeter is a type of wattmeter that is designed to measure the power in circuits with a low power factor. This type of wattmeter is used to measure the iron loss of a transformer. The iron loss of a transformer is the power that is lost in the transformer’s core due to eddy currents and hysteresis.
A unity power factor wattmeter is a type of wattmeter that is designed to measure the power in circuits with a unity power factor. This type of wattmeter is not used to measure the iron loss of a transformer.
A frequency meter is an instrument used to measure the frequency of an alternating current (AC) signal. The frequency of an AC signal is the number of cycles per second. The frequency of an AC signal is not used to measure the iron loss of a transformer.
Any type of wattmeter can be used to measure the power in a circuit. However, a low power factor wattmeter is the most accurate type of wattmeter to use to measure the iron loss of a transformer.