An electromagnet consists of a solenoid of length L L\,L with N N\,N turns of wire of resistance RRR, connected directly to a DC power supply of constant voltage V V\,V and negligible internal resistance.
Which of the following modifications will increase the magnetic flux density at the center of the solenoid?
Stretching the solenoid to double its length LLL while keeping the same wire and voltage VVV.
Cutting the solenoid in half to form a shorter solenoid of length L/2L/2L/2 with N/2N/2N/2 turns, and connecting it to the same voltage VVV.
Winding an identical second layer of wire over the solenoid (doubling the number of turns to 2N2N2N) and connecting the two layers in series to the same voltage VVV.
Replacing the wire with one of the same material and length but with half the wire diameter, keeping NNN and LLL the same, connected to the same voltage VVV.