Which electromagnet will produce the strongest magnetic field?

An electromagnet is a magnet that can be turned on and off. It consists of a central metal core, such as iron, and many wrappings of wire around the metal core. An electrical current sent through the wire wrappings causes the formation of a magnetic field. There are several ways to increase the strength of an electromagnet: increasing the number of wrappings, increasing the current of electricity, making the wrappings closer together, or increasing the diameter of the core.

Five electromagnets are shown above. The electrical power source for each electromagnet is an ordinary flashlight battery.