Which material's rod is usually used to make electromagnet? 

The correct answer is Soft iron.

Explanation:

• An electromagnet is a type of magnet in which the magnetic field is produced by an electric current.
• Electromagnets usually consist of wire wounds into a coil.
• Electromagnets are commonly made using ferromagnetic materials. 
• Iron is the most commonly used material for making the core of an electromagnet due to its high magnetic permeability.
• The electromagnets are made up of soft iron, which have low retentivity and low coercivity.
• It has high permeability value (μ).
• The magnetism of the electromagnets can be increased by placing a soft iron rod inside the solenoid.
• Other ferromagnetic materials like steel and nickel can also be used in constructing electromagnets, but soft iron is preferred due to its good or high magnetic properties, relatively low cost, and availability.
• Electromagnets are used in electric bells, loudspeakers, and telephone diaphragms.

Thus, Soft iron material's rod is usually used to make electromagnet.