Question

Which of the following would decrease the magnetic field around a wire?

Answer 1

The correct answer is
A. decreasing the current

In fact, the magnetic field produced by a current carrying wire is given by
$B(r) = \frac{\mu_0 I}{2 \pi r}$
where
$\mu_0$ is the vacuum permeability
I is the current in the wire
r is the distance from the wire at which the field is calculated

We see from the formula that the intensity of the field, B, is directly proportional to the current I, so if the current decreases, the magnetic field strength B decreases as well.