The three properties that determine the strength of any electro-magnet are intensity of current flow, number of wire turns in the coil and ____.

A) type of core material
B) circuit
C) magnetic field
D) voltage

The type of core material used in the electromagnet affects its magnetic properties, and therefore affects its strength. A ferromagnetic core material like iron is commonly used to increase the strength of the magnetic field. The other factors listed (circuit, magnetic field, voltage) are either unrelated or dependent on the first two properties listed (intensity of current flow and number of wire turns in coil).