Final answer:
Magnetostriction is a property unique to option a) ferromagnetic materials, such as iron, cobalt, nickel, and gadolinium, and involves changes in material dimensions in response to a magnetic field.
Step-by-step explanation:
Magnetostriction is a phenomenon that can only occur in option a) ferromagnetic materials.
This effect refers to the change in the dimensions of a ferromagnetic material when it experiences a magnetic field. Ferromagnetic materials, like iron, cobalt, nickel, and gadolinium, have their atomic magnetic dipoles strongly coupled and aligned, which allows them to be easily magnetized or made into permanent magnets. In contrast, other types of materials such as diamagnetic or paramagnetic materials do not exhibit this effect. Magnetostriction is directly tied to the magnetic domain structure of ferromagnetic materials, which is why it doesn't occur in nonmagnetic, anisotropic, or tubular materials if they are not ferromagnetic.