An air-cored solenoid of length L L\,L and radius r r\,r consists of N N\,N turns of wire of diameter ddd. The solenoid is connected across a power supply of constant DC voltage VVV.
Which of the following modifications will result in the greatest increase in the magnetic flux density B B\,B inside the solenoid?
Doubling the number of turns NNN by winding more of the same wire over the same length LLL and radius rrr.
Doubling the radius of the solenoid rrr while keeping NNN, LLL, and the wire diameter and material unchanged.
Using a wire of the same material but with twice the wire diameter ddd, keeping NNN, LLL, and the solenoid radius rrr constant.
Doubling both the length LLL of the solenoid and the number of turns NNN using the same wire.