What action will help reduce a magnetic field?

Moving the alternating current coil further away from the part will help reduce the magnetic field's strength at the location of interest. The magnetic field generated by a current-carrying coil decreases with distance, which is a fundamental principle of electromagnetism. As the coil is moved away from the test piece or part, the intensity of the magnetic field created by the coil diminishes due to the inverse-square law, which states that the strength of the field is inversely proportional to the square of the distance from the source.

In contrast, increasing the frequency of the alternating current coil would actually increase the magnetic field's complexity, potentially making it stronger in certain contexts. Moving the coil closer to the part would strengthen the magnetic field at that location, which is not what is desired for reducing the field. Switching from alternating to direct current could also change the nature of the magnetic field but doesn't inherently reduce its strength in the same way that increasing distance does. Thus, moving the coil farther away is the most straightforward and effective method for reducing the magnetic field.