The use of magnetic shielding around the exciting coil will generally:

The choice indicating that magnetic shielding around the exciting coil will decrease both the magnetic field extension and eddy current penetration is correct because magnetic shielding serves to contain and redirect magnetic fields. When a magnetic shield is placed around the coil, it absorbs some of the magnetic flux produced by the coil, which effectively reduces the range or extension of the magnetic field outside the coil.

As the magnetic field is weakened in the external environment due to the presence of the shield, the amount of induced eddy currents within conductive test materials also decreases. This phenomenon occurs because induced eddy currents arise from changing magnetic fields; if the field is substantially reduced or contained by the shield, fewer eddy currents are generated within the material being tested.

This understanding is fundamental in eddy current testing, where monitoring the presence and flow of these currents is critical for detecting material properties or flaws. Thus, the introduction of magnetic shielding is a method to control the magnetic influence, which in turn impacts the effectiveness of the eddy current testing process.