A starter solenoid switch has two windings that work together: a pull-in winding that must first drag the plunger inward, and a hold-in winding that only keeps the plunger seated once it has moved.

Dragging the plunger in against spring force demands a strong magnetic pull, which in turn requires the pull-in winding to carry a heavy current for that brief instant.

To pass such a heavy current with minimal resistance and voltage drop, the pull-in winding is wound from thick copper wire — copper for its high conductivity and a large cross-section to keep the resistance low.

Thick copper therefore lets a large current flow briefly to generate the powerful pull-in force needed to move the plunger.

The hold-in winding, by contrast, only has to maintain the plunger in place, so it needs just a small steady current and is wound from thin wire, which rules out the thin-wire option for the pull-in coil.

Aluminium has lower conductivity than copper, and silver, though an excellent conductor, is far too costly for a starter winding, so neither is used in place of thick copper here.

Hence, the answer is Thick Copper wire.