A no-load test is performed on a transformer to determine

The correct answer is: D. magnetising current

A no-load test is performed on a transformer to determine the magnetising current. The magnetising current is the current that flows through the transformer’s core when there is no load on the secondary winding. The magnetising current is responsible for creating the magnetic field that induces a voltage in the secondary winding.

The other options are incorrect because:

• Core loss is the power loss that occurs in the transformer’s core due to eddy currents and hysteresis. Core loss is determined by a short-circuit test.
• Copper loss is the power loss that occurs in the transformer’s windings due to the resistance of the wire. Copper loss is determined by an open-circuit test.
• Efficiency is the ratio of the output power of a transformer to the input power. Efficiency is determined by a load test.

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