ANS:B - partially filled

Ferromagnetic materials owe their properties to partially filled inner sub-shells. In ferromagnetic materials, such as iron, nickel, and cobalt, the magnetic properties arise from the alignment of magnetic moments associated with the electrons in the partially filled inner sub-shells of atoms. These partially filled inner sub-shells allow for the formation of magnetic domains and the alignment of magnetic moments within these domains, resulting in the overall magnetization of the material. Therefore, the correct choice is: partially filled