Which of the following statements is true about magnetic fields produced by current-carrying wires? the larger the current, the larger the magnetic field the smaller the current, the larger the magnetic field the longer the wire, the larger the magnetic field the longer the wire, the smaller the magnetic field

The larger the current flowing in a wire, the stronger the magnetic field
is that surrounds the wire. 

That's why, if you want to make an electromagnet stronger, one way to
do it is to add another battery.  By increasing the voltage, you'll increase
the current flowing in the coils of wire, and the electromagnet will be stronger.

Letter a, because when there are unlike charges they will always attract to each other.