Concept:To shield an instrument from external magnetic fields, a material with high magnetic permeability is used, as it provides a low reluctance path for magnetic field lines and prevents them from entering the protected region.
Explanation:Magnetic shielding requires a material that can divert magnetic flux around the instrument.
Soft ferromagnetic substances, such as soft iron or mu-metal, have very high permeability.
When these materials surround the instrument, they attract the magnetic field lines and confine them within their body.
This keeps the interior region almost free of the magnetic field.
Diamagnetic substances have very low permeability and can only weakly repel magnetic fields.
Paramagnetic substances have slightly higher permeability than diamagnetic substances, but still far too low for effective shielding.
Therefore, neither diamagnetic nor paramagnetic materials alone can provide sufficient shielding.
The practical and effective shielding material is a soft ferromagnetic substance.
Answer:Option A: soft ferromagnetic substance.