An electromagnet would have the greatest strength if its wire were wrapped around a core made of ferromagnetic material, such as iron, nickel, or cobalt.

Here's why:

* Ferromagnetic materials have a strong magnetic permeability, meaning they readily allow magnetic fields to pass through them.

* When a current flows through the wire, it creates a magnetic field.

* This magnetic field then induces a magnetic field in the ferromagnetic core, aligning the magnetic domains within the core.

* The combined magnetic field of the core and the wire significantly strengthens the electromagnet.

Other materials like wood or plastic have very low permeability and would have a minimal effect on the electromagnet's strength.