No-load on a transformer is carried out to determine

The correct answer is: B. magnetising current

A no-load test is carried out on a transformer to determine the magnetising current. This is the current that flows in the transformer when there is no load on the secondary winding. The magnetising current is used to calculate the core losses of the transformer.

The other options are incorrect because:

• Copper loss is the loss of power in the transformer due to the resistance of the windings. It is determined by measuring the current and voltage on the windings.
• Efficiency is the ratio of the output power of the transformer to the input power. It is determined by measuring the input and output voltages and currents.

I hope this helps! Let me know if you have any other questions.