The best choice for making a simple electromagnet would be:
A. metal rod for a core, copper wire, 9-volt battery
Using a metal rod as the core will enhance the magnetic field produced by the electromagnet because metal (especially ferromagnetic materials like iron) can be easily magnetized. The copper wire is necessary for conducting electricity, and the 9-volt battery provides the power source to create the electromagnetic field. The other options (glass, plastic, or wood) would not be effective as cores for an electromagnet since they are non-magnetic materials.