Verified answer

A solid copper rod , because air is an insulator , air is a very poor conductor of heat.

Any metal is a good conductor of heat and compared to air copper is much better.

This is because copper is full of de localised electrons , ie one that can move and conduct !!!

A hollow copper rod will conduct heat faster because there is less copper mass to heat up before the heat reaches the other end.

.

Good luck!

the solid copper rod would be a better conductor of heat as it has a larger surface area to conduct the heat though compared to the hollow rod.