The correct option is **B. Iron**. ### Detailed Explanation **1. Understanding Ferromagnetism:** Ferromagnetic materials are substances that can be magnetized; they have a strong magnetic response to an external magnetic field and can retain that magnetization even after the external field is removed. This property is due to the alignment of magnetic moments (tiny magnetic fields produced by the electrons) within the material. **2. Characteristics of Ferromagnetic Materials:** - **Magnetic Domains:** Ferromagnetic materials consist of regions called magnetic domains, where the magnetic moments of atoms are aligned in the same direction. When an external magnetic field is applied, these domains can grow in size and align with the field, resulting in a net magnetization. - **Retentivity:** After the external magnetic field is removed, some of these domains remain aligned, allowing the material to retain its magnetization. This property is known as retentivity or remanence. **3. Why Iron is Ferromagnetic:** Iron is a classic example of a ferromagnetic material. It has a high retentivity and can be easily magnetized. The atomic structure of iron allows for a significant number of unpaired electrons, which contribute to its strong magnetic properties. When iron is exposed to a magnetic field, the magnetic domains align, and even after the field is removed, many of these domains remain aligned, allowing iron to retain its magnetization. ### Analysis of Other Options **A. Copper:** - **Why it's not ferromagnetic:** Copper is a non-magnetic material. It does not have unpaired electrons that can contribute to a net magnetic moment. When exposed to a magnetic field, copper does not exhibit any significant magnetization and cannot retain any magnetic properties once the field is removed. **C. Aluminum:** - **Why it's not ferromagnetic:** Aluminum is classified as a paramagnetic material. It has some unpaired electrons, but its magnetic susceptibility is very weak. When an external magnetic field is applied, aluminum does not retain magnetization once the field is removed, as the magnetic domains do not align in a way that allows for permanent magnetization. **D. Gold:** - **Why it's not ferromagnetic:** Gold is also a non-magnetic material. Similar to copper, it does not have unpaired electrons that would allow it to exhibit ferromagnetic properties. Gold does not respond to magnetic fields in a way that would allow it to retain magnetization. ### Summary of Key Points - **Ferromagnetic materials** can retain magnetization after the external magnetic field is removed due to the alignment of magnetic domains. - **Iron** is a ferromagnetic material with high retentivity, making it capable of being magnetized and retaining that magnetization. - **Copper, aluminum, and gold** are not ferromagnetic; they either do not respond to magnetic fields or do not retain magnetization after the field is removed. ### Revision Summary - Ferromagnetic materials retain magnetization after an external field is removed. - Iron is a prime example of a ferromagnetic material due to its structure and high retentivity. - Copper, aluminum, and gold do not exhibit ferromagnetic properties and cannot retain magnetization.