Answer: B. increase the number of coils of insulated wire.
How do you increase the strength of an electromagnet?
There are three main ways of doing this. You can add more turns to the coil, increase the current flowing through the coil, or put a piece of metal through the coil.
There is already a piece of metal, the iron nail, through the coil so this isn't the answer we're looking for. The only answer that lines up with what we know is;
B. increase the number of coils of insulated wire.