The option that would make the weakest electromagnet is:
a. Using a plastic core instead of an iron core.
Plastic is a non-magnetic material and does not enhance the magnetic field, while iron is a ferromagnetic material that significantly increases the strength of the electromagnet. The other options (using fewer batteries, a smaller core, or fewer coils) would also weaken the electromagnet, but using a plastic core fundamentally means losing the core's ability to amplify the magnetic field.