Magnetic permeability quantifies a material's capacity to facilitate magnetic field lines. This property is often assessed relative to the permeability of free space, represented as \( \mu_0 \).- Iron: As a ferromagnetic material, its magnetic permeability significantly exceeds that of free space.- Aluminium: Classified as a paramagnetic material, its permeability is marginally higher than free space, though less so than ferromagnetic substances.- Sodium: Also a paramagnetic material, its magnetic permeability is slightly above that of free space, comparable to aluminium.- Copper: Considered a non-magnetic material, its magnetic permeability is infinitesimally lower than free space due to a subtle diamagnetic characteristic.Therefore, the correct selection is:\[\boxed{D} \text{ Copper}\]