Which of the following would increase the conductivity of a connecting wire?

increase the diameter of the wire,

increase the length of the wire,

decrease the temperature of the wire,

or switch from steel wire to gold wire?

All but length I suppose. Diameter would definitely increase the conductivity of a wire, that's why jumper cables are so big, to conduct very large amounts of electricity. Also decreasing temperature allows for a faster flow of electrons causing better conductivity. Finally, Gold is very, very conductive and would be used in wires everywhere if it weren't so expensive (that's why copper is used a lot instead) also gold is quite delicate (wouldn't want to bend it back and forth a lot like most wires) but simply put, yes. Gold would also very much up the conductivity.