C

Step 1: Shielding of magnetic instruments requires materials that can effectively redirect magnetic field lines.
Step 2: Soft iron is known for its high magnetic permeability, which allows it to easily absorb and redirect magnetic fields.
Step 3: While materials like teak wood and plastic are non-conductive and will not provide effective shielding against magnetic fields, metals with high conductivity do not specifically redirect magnetic fields as effectively as soft iron.
Therefore, the best option for effective shielding from outside magnetic fields is soft iron of high permeability.