Which feature of transformer cores helps reduce eddy current losses?

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Eddy currents are looping currents that form in conductive materials when magnetic flux changes, and they turn some of the magnetic energy into heat. The way to cut those losses in a transformer core is to break up those loops so the currents can’t circulate freely through a large cross-section. That’s exactly what laminating the core does: the core is built from many thin sheets of silicon steel, each insulated from the next. The insulation between sheets prevents large, continuous current loops, so any eddy currents that do form are confined to small, short paths within individual sheets. Because the loop area is small and the current is restricted, the energy lost to these currents drops dramatically. Silicon steel helps by having higher resistivity than plain iron, which further slows the currents.

A solid iron core would let big eddy currents flow, increasing losses. An air core wouldn’t provide the solid magnetic path needed for efficient transformer operation. Copper windings relate to the coils around the core, not to reducing eddy currents in the core itself.