The open-circuit test on a transformer is performed to determine the core losses. These losses are primarily due to hysteresis and eddy currents within the transformer's core material. During the open-circuit test, the high-voltage side of the transformer is energized at rated voltage, while the low-voltage side is left open. This essentially eliminates the copper losses (I²R losses in the windings) as there's negligible current flowing in the secondary winding. The input power measured under these conditions is almost entirely consumed by the core itself, representing the core losses.

Therefore, the correct answer is Core loss.

Let's examine why the other options are incorrect:

• Friction loss: Friction losses are mechanical losses associated with bearings and other moving parts in a transformer. The open-circuit test doesn't measure these mechanical losses.
• Copper loss: Copper losses are I²R losses in the windings, directly proportional to the square of the current. The open-circuit test minimizes current flow, and hence these losses are negligible.
• Winding loss: This is another term for copper loss, and is negligible during the open-circuit test.