Which of the following materials is classified as ferromagnetic?

Ferromagnetic materials are those that exhibit strong magnetic properties and can be magnetized easily. They have a high susceptibility to magnetization, meaning they can become magnetized in the presence of an external magnetic field and retain that magnetization when the field is removed.

Iron is a classic example of a ferromagnetic material. Its atomic structure features unpaired electrons that contribute to its magnetic moment, allowing the atoms to align in the direction of an applied magnetic field. This unique property is what makes iron, along with other ferromagnetic materials like cobalt and nickel, essential in various applications, such as the manufacturing of magnets and transformers.

The other materials listed do not exhibit ferromagnetic properties. Platinum is considered paramagnetic, exhibiting weak attraction to magnetic fields but not retaining magnetization. Copper is diamagnetic, repelling magnetic fields weakly. Brass, being an alloy primarily of copper and zinc, shares similar non-magnetic characteristics. These distinctions highlight why iron is categorized as ferromagnetic, making it the correct answer in this context.