Looking specifically at the geometry of a laminated core versus a solid one clarifies why lamination targets only one of these losses.
- Copper loss: This loss lives entirely in the winding wires, laminating the iron core changes nothing about the copper conductor's resistance, so this loss is untouched by lamination.
- Hysteresis loss: This loss depends on the magnetic properties of the core material itself, not its physical shape or slicing, so cutting the core into layers doesn't change how strongly its magnetic domains resist reorientation.
- Friction loss: A transformer core is a static block of material with no rotating or sliding parts, so there is no mechanical friction present to begin with, laminating a stationary block cannot reduce a loss that was never there.
- Eddy current loss: A solid iron core presents a large, unbroken cross-section for induced currents to loop through, offering very little resistance to those loops. Cutting the same core into thin sheets, each insulated from its neighbours by a thin varnish layer, forces any induced current to stay confined within a single thin sheet's small cross-section, which raises the resistance those currents encounter and shrinks the loops they can form, cutting the associated power loss sharply.
The geometry of thin, insulated layers specifically interrupts the paths that circulating induced currents would otherwise take.
Therefore, the correct answer is eddy current loss.