Which materials are classified as ferromagnetic based on their permeabilities compared to free space? 

The correct option is 4

Concept:

• Ferromagnetic materials are characterized by their high permeability to magnetic fields, that is, their ability to concentrate magnetic field lines. Permeability refers to a material's property that determines the ease with which a magnetic field can pass through it.
• Ferromagnetic materials, such as iron, nickel, cobalt, and certain alloys or rare-earth metals, have a permeability much much greater than that of free space. This high permeability (hundreds and thousands of times greater than free space) allows these materials to become strongly magnetized when placed in a magnetic field. In fact, they can remain magnetized even after the original magnetic field is removed, which is what makes them so useful in the creation of permanent magnets and in various types of magnetic storage media.
• The relative permeability of ferromagnetic materials can vary from several hundreds to several thousands of times the permeability of free space. This makes option 4) the correct choice:
• Materials with permeabilities hundreds and thousands of times greater than that of free space