Which of the following actions will decrease the strength of the magnetic field of an electromagnet?

The correct answer is: All of the above

• The magnetic field inside a current carrying solenoid is given by 
• $B=\mu nI$
• where $n$ is the number of turns per unit length in the solenoid
• $I$ is the current through the wire
• Hence decreasing current or number of loops per meter would decrease the magnetic field in solenoid
• The iron core being a magnet itself is magnetized hence it contributes flux to the solenoid's flux, hence the magnetic field strength. Hence removing iron core would also decrease magnetic field strength
• Correct option is D).