To increase the strength of the electromagnet, the student can select the following options:
A. He can use a thicker piece of copper wire. - A thicker wire has a lower resistance which can allow more current to flow, increasing the magnetic field strength.
E. He can replace the 1.5-volt battery with a battery of higher voltage. - A higher voltage battery will increase the current flowing through the wire, which strengthens the electromagnet.
F. He can use a longer piece of wire that wraps more times around the bolt. - Increasing the number of coils creates a larger magnetic field, enhancing the strength of the electromagnet.
Options B, C, and D do not effectively contribute to increasing the magnetic strength.