The correct answer is: B. Copper losses
Open circuit test on transformers is conducted to determine the copper losses. Copper losses are the losses that occur in the windings of the transformer due to the resistance of the copper wire. These losses are proportional to the square of the current flowing through the windings.
Hysteresis losses are the losses that occur in the core of the transformer due to the magnetic hysteresis effect. This effect is caused by the alternating magnetic field that is produced by the current flowing through the windings. Hysteresis losses are proportional to the frequency of the alternating current.
Eddy current losses are the losses that occur in the core of the transformer due to the eddy currents that are induced in the core by the alternating magnetic field. These losses are proportional to the square of the frequency of the alternating current.
The open circuit test is conducted by applying a voltage to the primary winding of the transformer while the secondary winding is open circuited. The current that flows through the primary winding is then measured. The copper losses are then calculated using the following formula:
$P_c = I^2R$
where:
$P_c$ is the copper losses
$I$ is the current flowing through the primary winding
$R$ is the resistance of the primary winding
The open circuit test is a simple and inexpensive test that can be used to determine the copper losses of a transformer. The test can be conducted on any type of transformer, regardless of its size or power rating.